NFC.
Updated 8 regression tests to use -mattr instead of -mcpu flag as follows:
-mcpu=knl --> -mattr=+avx512f
-mcpu=skx --> -mattr=+avx512f,+avx512bw,+avx512vl,+avx512dq
The updates are as part of the preparation of a large commit to add all instruction scheduling for the SKX target.
Please first clean these extra comments in a separate commit and rebase