Since https://crrev.com/c/1112003, the memory size is stored as size_t
instead of uint32_t in order to support 4GB memories.
Loading it as uint32_t only works on little-endian systems, and only for
memory sizes <4GB. This CL fixes this to load and process the memory
size as pointer-sized value.
Additional platform-specific methods are added to perform a shift by a
constant value. This can be reused to improve the generated code for
other shifts.

R=titzer@chromium.org

Bug: v8:8130, v8:6600
Change-Id: Ifa688a3ed0e2809190571f24bdf47a7f53880b3d
Reviewed-on: https://chromium-review.googlesource.com/1203950
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55609}