i386: Refactor m512-check.h

After AVX10 introduction, we still want to use AVX512 helper functions
to avoid duplicate code. In order to reuse them, we need to do some refactor
to make sure each function define happen under correct ISA to avoid ABI
warnings.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/m512-check.h: Wrap the function define with
	correct vector size.