Add tgmath.h macros for narrowing functions.

When adding some of the TS 18661 narrowing functions for glibc 2.28, I
deferred adding corresponding <tgmath.h> support because of unresolved
questions about the specification for those type-generic macros,
especially in relation to _FloatN and _FloatNx types.

Those issues are now clarified in the response to Clarification
Request 13 to TS 18661-3, and this patch adds the deferred tgmath.h
support.  As with other tgmath.h macros, there are fairly
straightforward implementations based on __builtin_tgmath for GCC 8
and later, which result in exactly the right function being called in
each case, and more complicated implementations for GCC 7 and earlier,
which generally result in a function being called whose arguments have
the right format (i.e. an alias for the right function), but which
might not be exactly the function name specified by TS 18661.

In one case with older compilers (f32x* macros, where the type
_Float64x exists and all the arguments have type _Float32 or
_Float32x), there is a further relaxation and the function called may
have arguments narrower than the one specified by the TS, but still
wide enough to represent the arguments exactly, so the result of the
call is unchanged (as this does not affect any case where rounding of
integer arguments might be involved).  With GCC 6 or before this is
inherently unavoidable (but still harmless and not detectable by how
the compiled program behaves, unless it redefines the functions in
question like the testcases do) because _Float32x and _Float64 are
both typedefs for double in that case but the specified semantics
result in different functions, with different argument formats, being
called for those two argument types.

Tests for the new macros are handled through gen-tgmath-tests.py,
which deals with the special-case handling for older GCC.

Tested as follows: with the full glibc testsuite on x86_64 and x86
(with GCC 6, 7 and 8); with the math/ tests on aarch64 and arm (with
GCC 6, 7 and 8); with build-many-glibcs.py (with GCC 6, 7 and 9).

	* math/tgmath.h [__HAVE_FLOAT128X]: Give error.
	[(__HAVE_FLOAT64X && !__HAVE_FLOAT128)
	|| (__HAVE_FLOAT128 && !__HAVE_FLOAT64X)]: Likewise.
	(__TGMATH_2_NARROW_F): Likewise.
	(__TGMATH_2_NARROW_D): New macro.
	(__TGMATH_2_NARROW_F16): Likewise.
	(__TGMATH_2_NARROW_F32): Likewise.
	(__TGMATH_2_NARROW_F64): Likewise.
	(__TGMATH_2_NARROW_F32X): Likewise.
	(__TGMATH_2_NARROW_F64X): Likewise.
	[__HAVE_BUILTIN_TGMATH] (__TGMATH_NARROW_FUNCS_F): Likewise.
	[__HAVE_BUILTIN_TGMATH] (__TGMATH_NARROW_FUNCS_F16): Likewise.
	[__HAVE_BUILTIN_TGMATH] (__TGMATH_NARROW_FUNCS_F32): Likewise.
	[__HAVE_BUILTIN_TGMATH] (__TGMATH_NARROW_FUNCS_F64): Likewise.
	[__HAVE_BUILTIN_TGMATH] (__TGMATH_NARROW_FUNCS_F32X): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (fadd): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (dadd): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (fdiv): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (ddiv): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (fmul): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (dmul): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (fsub): Likewise.
	[__GLIBC_USE (IEC_60559_BFP_EXT_C2X)] (dsub): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT16] (f16add):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT16] (f16div):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT16] (f16mul):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT16] (f16sub):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32] (f32add):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32] (f32div):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32] (f32mul):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32] (f32sub):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64
	&& (__HAVE_FLOAT64X || __HAVE_FLOAT128)] (f64add): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64
	&& (__HAVE_FLOAT64X || __HAVE_FLOAT128)] (f64div): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64
	&& (__HAVE_FLOAT64X || __HAVE_FLOAT128)] (f64mul): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64
	&& (__HAVE_FLOAT64X || __HAVE_FLOAT128)] (f64sub): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32X] (f32xadd):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32X] (f32xdiv):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32X] (f32xmul):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT32X] (f32xsub):
	Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64X
	&& (__HAVE_FLOAT128X || __HAVE_FLOAT128)] (f64xadd): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64X
	&& (__HAVE_FLOAT128X || __HAVE_FLOAT128)] (f64xdiv): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64X
	&& (__HAVE_FLOAT128X || __HAVE_FLOAT128)] (f64xmul): Likewise.
	[__GLIBC_USE (IEC_60559_TYPES_EXT) && __HAVE_FLOAT64X
	&& (__HAVE_FLOAT128X || __HAVE_FLOAT128)] (f64xsub): Likewise.
	* math/gen-tgmath-tests.py (Type): Add members
	non_standard_real_argument_types_list, long_double_type,
	complex_float64_type and float32x_ext_type.
	(Type.__init__): Set the new members.
	(Type.floating_type): Add new argument floatn.
	(Type.real_floating_type): Likewise.
	(Type.can_combine_types): Likewise.
	(Type.combine_types): Likewise.
	(Type.init_types): Create internal Float32x_ext type.
	(Tests.__init__): Define Float32x_ext in generated C code.
	(Tests.add_tests): Handle narrowing functions.
	(Tests.add_all_tests): Likewise.
	(Tests.tests_text): Allow variation in mant_dig for narrowing
	functions with compilers before GCC 8.
	* math/Makefile (tgmath3-narrow-types): New variable.
	(tgmath3-narrow-macros): Likewise.
	(tgmath3-macros): Add $(tgmath3-narrow-macros).

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