Fix Unix ARM JIT_MemCpy and JIT_MemSet (#21141)

Fix Unix ARM JIT_MemCpy and JIT_MemSet (#21141)

The functions were incorrectly using 4 byte loads to probe for
the address validity. While the comment on JIT_MemCpy requires
4 byte aligned address, it doesn't match the way JIT uses it and
the Windows version of the function works with unaligned addresses
too.
This bug was discovered as a crash in an application where the
JIT_MemCpy was called with count=2 and an address that was two
bytes below the end of a memory page where the following page
was not mapped.