x86: Optimize memrchr-evex.S

Optimizations are:
1. Use the fact that lzcnt(0) -> VEC_SIZE for memchr to save a branch
   in short string case.
2. Save several instructions in len = [VEC_SIZE, 4 * VEC_SIZE] case.
3. Use more code-size efficient instructions.
	- tzcnt ...     -> bsf ...
	- vpcmpb $0 ... -> vpcmpeq ...

Code Size Changes:
memrchr-evex.S      :  -29 bytes

Net perf changes:

Reported as geometric mean of all improvements / regressions from N=10
runs of the benchtests. Value as New Time / Old Time so < 1.0 is
improvement and 1.0 is regression.

memrchr-evex.S      : 0.949 (Mostly from improvements in small strings)

Full results attached in email.

Full check passes on x86-64.