Fix test-math-vector-sincos.h aliasing.

x86_64 libmvec tests have been failing to build lately with GCC
mainline with -Wuninitialized errors, and Markus Trippelsdorf traced
this to an aliasing issue
<https://sourceware.org/ml/libc-alpha/2017-03/msg00169.html>.

This patch fixes the aliasing issue, so that the vectors-of-pointers
are initialized using a union instead of pointer casts.  This also
fixes the testsuite build failures with GCC mainline.

Tested for x86_64 (full testsuite with GCC 6; testsuite build with GCC
mainline with build-many-glibcs.py).

	* sysdeps/x86/fpu/test-math-vector-sincos.h (INIT_VEC_PTRS_LOOP):
	Use a union when storing pointers.
	(VECTOR_WRAPPER_fFF_2): Do not take address of integer vector and
	cast result when passing to INIT_VEC_PTRS_LOOP.
	(VECTOR_WRAPPER_fFF_3): Likewise.
	(VECTOR_WRAPPER_fFF_4): Likewise.

(cherry picked from commit ffe308e4)