x86: Add support for VEC_SIZE == 64 in strcmp-evex.S impl

Unused at the moment, but evex512 strcmp, strncmp, strcasecmp{l}, and
strncasecmp{l} functions can be added by including strcmp-evex.S with
"x86-evex512-vecs.h" defined.

In addition save code size a bit in a few places.

1. tzcnt ...         -> bsf ...
2. vpcmp{b|d} $0 ... -> vpcmpeq{b|d}

This saves a touch of code size but has minimal net affect.

Full check passes on x86-64.