Check AVX-512 assembler support first

It checks AVX-512 assembler support first and sets libc_cv_cc_avx512 to
$libc_cv_asm_avx512, instead of yes.  GCC won't support AVX-512 if
assembler doesn't support it.

	* sysdeps/x86_64/configure.ac: Check AVX-512 assembler support
	first.  Disable AVX-512 GCC support if assembler doesn't support
	it.
	* sysdeps/x86_64/configure: Regenerated.