int128: Check BITS_PER_MP_LIMB == 32 instead of __WORDSIZE == 32

int128: Check BITS_PER_MP_LIMB == 32 instead of __WORDSIZE == 32

commit 8cd6efca5b3796193ef3ff60d9dbf6e5572b2b73
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Thu Nov 20 15:30:06 2025 -0300

    Add add_ssaaaa and sub_ssaaaa to gmp-arch.h

checks __WORDSIZE == 32 to decide if int128 should be used, which breaks
x32 which has int128 and __WORDSIZE == 32.  Check BITS_PER_MP_LIMB == 32,
instead of __WORDSIZE == 32.  This fixes BZ #33677.

Tested on x32, x86-64 and i686.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>