x86-64: Fix SSE2 memcmp and SSSE3 memmove for x32

Clear the upper 32 bits in RDX (memory size) for x32 to fix

FAIL: string/tst-size_t-memcmp
FAIL: string/tst-size_t-memcmp-2
FAIL: string/tst-size_t-memcpy
FAIL: wcsmbs/tst-size_t-wmemcmp

on x32 introduced by

8804157a x86: Optimize memcmp SSE2 in memcmp.S
26b24783 x86: Reduce code size of mem{move|pcpy|cpy}-ssse3

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit 8ea20ee5)