x86-64: Add sincosf with vector FMA

Since the x86-64 assembly version of sincosf is higly optimized with
vector instructions, there isn't much room for improvement.  However
s_sincosf.c written in C with vector math and intrinsics can be
optimized by GCC with FMA.

On Skylake, bench-sincosf reports performance improvement:

           Assembly       FMA         improvement
max        104.042       101.008         3%
min        9.426         8.586           10%
mean       20.6209       18.2238         13%

	* sysdeps/x86_64/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add s_sincosf-sse2 and s_sincosf-fma.
	(CFLAGS-s_sincosf-fma.c): New.
	* sysdeps/x86_64/fpu/multiarch/s_sincosf-fma.c: New file.
	* sysdeps/x86_64/fpu/multiarch/s_sincosf-sse2.S: Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_sincosf.c: Likewise.
	* sysdeps/x86_64/fpu/s_sincosf.S: Don't add alias if
	__sincosf is defined.