x86: Optimize strlen-avx2.S

No bug. This commit optimizes strlen-avx2.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>