Add SSE4.1 trunc, truncf (bug 20142).

This patch adds SSE4.1 versions of trunc and truncf, using the roundsd
/ roundss instructions, similar to the versions of ceil, floor, rint
and nearbyint functions we already have.  In my testing with the glibc
benchtests these are about 30% faster than the C versions for double,
20% faster for float.

Tested for x86_64.

	[BZ #20142]
	* sysdeps/x86_64/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add s_trunc-c, s_truncf-c, s_trunc-sse4_1 and s_truncf-sse4_1.
	* sysdeps/x86_64/fpu/multiarch/s_trunc-c.c: New file.
	* sysdeps/x86_64/fpu/multiarch/s_trunc-sse4_1.S: Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_trunc.c: Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_truncf-c.c: Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_truncf-sse4_1.S: Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_truncf.c: Likewise.