Add stubs for pseudocode functions "MemA[]" amd "MemU[]", corresponding to aligned
and unaligned memory accesses.  The new stub functions are MemARead, MemAWrite,
MemURead, and MemUWrite.  At the moment these stubs just call ReadMemoryUnsigned or
WriteMemoryUnsigned, but we can fill them out further later if we decide we need
more accurate emulation of the memory system.

Replaced all the direct calls to ReadMemoryUnsigned and WriteMemoryUnsigned in
EmulateInstructionARM.cpp with calls to the appropriate new stub function.

llvm-svn: 125766