Hi:
  This patch is to adjust testcases which failed the regression test
when gcc is built with -march=skylake-avx512.
  Also add runtime check for AVX512 tests.

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.


-- 
BR,
Hongtao