x86: Shrink code size of memchr-avx2.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: 59 bytes

There are no major changes in the benchmarks.
Geometric Mean of all benchmarks New / Old: 0.967

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

(cherry picked from commit 6dcbb7d9)

x86: Fix page cross case in rawmemchr-avx2 [BZ #29234]

commit 6dcbb7d9
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Mon Jun 6 21:11:33 2022 -0700

    x86: Shrink code size of memchr-avx2.S

Changed how the page cross case aligned string (rdi) in
rawmemchr. This was incompatible with how
`L(cross_page_continue)` expected the pointer to be aligned and
would cause rawmemchr to read data start started before the
beginning of the string. What it would read was in valid memory
but could count CHAR matches resulting in an incorrect return
value.

This commit fixes that issue by essentially reverting the changes to
the L(page_cross) case as they didn't really matter.

Test cases added and all pass with the new code (and where confirmed
to fail with the old code).
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit 2c9af842)