x86: Optimize strlen-evex.S

No bug. This commit optimizes strlen-evex.S. The
optimizations are mostly small things but they add up to roughly
10-30% performance improvement for strlen. The results for strnlen are
bit more ambiguous. test-strlen, test-strnlen, test-wcslen, and
test-wcsnlen are all passing.

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit 4ba65586)