x86: Optimize strspn in strspn-c.c

Use _mm_cmpeq_epi8 and _mm_movemask_epi8 to get strlen instead of
_mm_cmpistri. Also change offset to unsigned to avoid unnecessary
sign extensions.

geometric_mean(N=20) of all benchmarks that dont fallback on
sse2; New / Original: .901

All string/memory tests pass.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>