Fix x86, x86_64 fmax, fmin sNaN handling, add tests (bug 20947).

Various fmax and fmin function implementations mishandle sNaN
arguments:

(a) When both arguments are NaNs, the return value should be a qNaN,
but sometimes it is an sNaN if at least one argument is an sNaN.

(b) Under TS 18661-1 semantics, if either argument is an sNaN then the
result should be a qNaN (whereas if one argument is a qNaN and the
other is not a NaN, the result should be the non-NaN argument).
Various implementations treat sNaNs like qNaNs here.

This patch fixes the x86 and x86_64 versions (ignoring float and
double for 32-bit x86 given the inability to reliably avoid the sNaN
turning into a qNaN before it gets to the called function).  Tests of
sNaN inputs to these functions are added.

Note on architecture versions I haven't changed for this issue:
AArch64 already gets this right (it uses a hardware instruction with
the correct semantics for both quiet and signaling NaNs) and does not
need changes.  It's possible Alpha, IA64, SPARC might need changes
(this would be shown by the testsuite if so).

Tested for x86_64 and x86 (both i686 and i586 builds, to cover the
different x86 implementations).

	[BZ #20947]
	* sysdeps/i386/fpu/s_fmaxl.S (__fmaxl): Add the arguments when
	either is a signaling NaN.
	* sysdeps/i386/fpu/s_fminl.S (__fminl): Likewise.  Make code
	follow fmaxl more closely.
	* sysdeps/i386/i686/fpu/s_fmaxl.S (__fmaxl): Add the arguments
	when either is a signaling NaN.
	* sysdeps/i386/i686/fpu/s_fminl.S (__fminl): Likewise.
	* sysdeps/x86_64/fpu/s_fmax.S (__fmax): Likewise.
	* sysdeps/x86_64/fpu/s_fmaxf.S (__fmaxf): Likewise.
	* sysdeps/x86_64/fpu/s_fmaxl.S (__fmaxl): Likewise.
	* sysdeps/x86_64/fpu/s_fmin.S (__fmin): Likewise.
	* sysdeps/x86_64/fpu/s_fminf.S (__fminf): Likewise.
	* sysdeps/x86_64/fpu/s_fminl.S (__fminl): Likewise.
	* math/libm-test.inc (fmax_test_data): Add tests of sNaN inputs.
	(fmin_test_data): Likewise.

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