Only use Platform_MemRead/Write() for target memory

Routines like writeHexBufferToHalfWordMemory() and
writeHexBufferToHalfWordMemory() were using these Platform APIs to even
access local stack variables within the debugger.  This caused problems
for pinkySim where the target memory is in a different address space
from the debugger stub.
Add code missed in last commit

I also need to handle the writes from binary buffers in a similar
fashion as what I did with the previous change with respect to writes
from hex buffers.