ENH: use memchr for unit stride all/any

Improves performance by a factor of two with GNU libc.
Requires that boolean true == 1 for any.

The obsolete handvectorized code in simd.inc.src could be improved to
the same performance via unrolling once and using pminub/pmaxub to save
one slow pmovmskb instruction but using the libc routine has the
advantage that it will make best use of the underlying hardware no
matter how numpy was compiled.