Support sNaN testing in libm-test.inc.

Support sNaN testing in libm-test.inc.

This patch adds support in libm-test.inc for tests with signaling NaN
arguments.  gen-libm-test.pl is made to set a flag TEST_SNAN for such
tests, so that they can be disabled automatically when sNaN testing
isn't supported for a given type (at present, for float and double on
32-bit x86 because it's unpredictable when a value might be loaded
into a floating-point register and so automatically converted to long
double with sNaNs converted to quiet NaNs).  -fsignaling-nans is used
where needed.

Tests are added for classification macros, as a starting point; this
is deliberately more conservative than Thomas's patch
<https://sourceware.org/ml/libc-ports/2013-04/msg00008.html>, to allow
more tests to be added, and issues exposed fixed, bit by bit.

Tested for x86_64 and x86.

	* math/libm-test.inc: Update comment about NaN testing.
	(TEST_SNAN): New macro.
	(snan_value): Likewise.
	(enable_test): Disable tests of sNaNs when SNAN_TESTS fails.
	(fpclassify_test_data): Add sNaN tests.
	(isfinite_test_data): Likewise.
	(isinf_test_data): Likewise.
	(isnan_test_data): Likewise.
	(isnormal_test_data): Likewise.
	(issignaling_test_data): Likewise.
	(signbit_test_data): Likewise.
	* math/gen-libm-test.pl (%beautify): Add snan_value.
	(show_exceptions): Add argument $test_snan.
	(parse_args): Handle snan_value as non-finite.  Update call to
	show_exceptions.
	* math/Makefile (libm-test-no-inline-cflags): Add
	-fsignaling-nans.