x86: Shrink code size of memchr-evex.S

This is not meant as a performance optimization. The previous code was
far to liberal in aligning targets and wasted code size unnecissarily.

The total code size saving is: 64 bytes

There are no non-negligible changes in the benchmarks.
Geometric Mean of all benchmarks New / Old: 1.000

Full xcheck passes on x86_64.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>