x86: Fallback {str|wcs}cmp RTM in the ncmp overflow case [BZ #29127]

Re-cherry-pick commit c6272098 for strcmp-avx2.S change which was
omitted in intial cherry pick because at the time this bug was not
present on release branch.

Fixes BZ #29127.

In the overflow fallback strncmp-avx2-rtm and wcsncmp-avx2-rtm would
call strcmp-avx2 and wcscmp-avx2 respectively. This would have
not checks around vzeroupper and would trigger spurious
aborts. This commit fixes that.

test-strcmp, test-strncmp, test-wcscmp, and test-wcsncmp all pass on
AVX2 machines with and without RTM.

Co-authored-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit c6272098)