Fix crash at startup in SSE4.2 code when running a 32 bits Windows build

Fix crash at startup in SSE4.2 code when running a 32 bits Windows build

There is no guarantee that a g_malloc'ed memory block will be aligned on a 128 bits boundary
Instead use a static variable definition (at the cost of exposing the HAVE_SSE4_2 compilation flag in ws_mempbrk.h)

Change-Id: I661bf479a9d458d64c96bafc940c519d29a4780b
Reviewed-on: https://code.wireshark.org/review/7070
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>