Adjust testcase.

Adjust testcase.

gcc/testsuite/ChangeLog:
	PR target/96246
	PR target/96855
	PR target/96856
	PR target/96857
	* g++.target/i386/avx512bw-pr96246-2.C: Add runtime check for
	AVX512BW.
	* g++.target/i386/avx512vl-pr96246-2.C: Add runtime check for
	AVX512BW and AVX512VL
	* g++.target/i386/avx512f-helper.h: New header.
	* gcc.target/i386/pr92658-avx512f.c: Add
	-mprefer-vector-width=512 to avoid impact of different default
	mtune which gcc is built with.
	* gcc.target/i386/avx512bw-pr95488-1.c: Ditto.
	* gcc.target/i386/pr92645-4.c: Add -mno-avx512f to avoid
	impact of different default march which gcc is built with.