Provide a C++ version of iseqsig (bug 22377)

In C++ mode, __MATH_TG cannot be used for defining iseqsig, because
__MATH_TG relies on __builtin_types_compatible_p, which is a C-only
builtin.  This is true when float128 is provided as an ABI-distinct type
from long double.

Moreover, the comparison macros from ISO C take two floating-point
arguments, which need not have the same type.  Choosing what underlying
function to call requires evaluating the formats of the arguments, then
selecting which is wider.  The macro __MATH_EVAL_FMT2 provides this
information, however, only the type of the macro expansion is relevant
(actually evaluating the expression would be incorrect).

This patch provides a C++ version of iseqsig, in which only the type of
__MATH_EVAL_FMT2 (__typeof or decltype) is used as a template parameter
for __iseqsig_type.  This function calls the appropriate underlying
function.

Tested for powerpc64le and x86_64.

	[BZ #22377]
	* math/Makefile [C++] (tests): Add test for iseqsig.
	* math/math.h [C++] (iseqsig): New implementation, which does
	not rely on __MATH_TG/__builtin_types_compatible_p.
	* math/test-math-iseqsig.cc: New file.
	* sysdeps/powerpc/powerpc64le/Makefile
	(CFLAGS-test-math-iseqsig.cc): New variable.

1 files changed, 1 insertions, 1 deletions

diff --git a/math/Makefile b/math/Makefile
index cba9ce9405..8978f2ef96 100644
--- a/math/Makefile
+++ b/math/Makefile
@@ -220,7 +220,7 @@ tests-static += atest-exp atest-sincos atest-exp2
 
 ifneq (,$(CXX))
 tests += test-math-isinff test-math-iszero test-math-issignaling \
-	 test-math-iscanonical test-math-cxx11
+	 test-math-iscanonical test-math-cxx11 test-math-iseqsig
 endif
 
 ifneq (no,$(PERL))