Fix modf (sNaN) (bug 20240).

Various modf implementations return sNaN (both outputs) for sNaN
input.  In fact they contain code to convert sNaN to qNaN for both
outputs, but the way this is done is multiplying by 1.0 (for a wider
range of inputs that includes NaNs as well as numbers with exponent
large enough to ensure that they are integers), and that
multiplication by 1.0 is optimized away by GCC in the absence of
-fsignaling-nans, unlike other operations on NaNs used for this
purpose that are not no-ops for non-sNaN input.  This patch arranges
for those files to be built with -fsignaling-nans so that this
existing code is effective as intended.

Tested for x86_64 and x86.

	[BZ #20240]
	* math/Makefile (CFLAGS-s_modf.c): New variable.
	(CFLAGS-s_modff.c): Likewise.
	(CFLAGS-s_modfl.c): Likewise.
	* math/libm-test.inc (modf_test_data): Add sNaN tests.

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