Remove -mavx2 configure tests.

There are configure tests for the -mavx2 compiler option.  AVX2
support was added in GCC 4.7, so these tests are now obsolete; this
patch removes them.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	* sysdeps/i386/configure.ac (libc_cv_cc_avx2): Remove configure
	test.
	* sysdeps/i386/configure: Regenerated.
	* sysdeps/x86_64/configure.ac (libc_cv_cc_avx2): Remove configure
	test.
	* sysdeps/x86_64/configure: Regenerated.
	* config.h.in (HAVE_AVX2_SUPPORT): Remove #undef.
	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	memset-avx2 unconditionally instead of conditionally on
	[$(config-cflags-avx2) = yes].
	* sysdeps/x86_64/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list) [HAVE_AVX2_SUPPORT]: Make code
	unconditional.
	* sysdeps/x86_64/multiarch/memset.S [HAVE_AVX2_SUPPORT]: Likewise.
	* sysdeps/x86_64/multiarch/memset_chk.S
	[IS_IN (libc) && SHARED && HAVE_AVX2_SUPPORT]: Change conditional
	to [IS_IN (libc) && SHARED].