Add memory-map.xml and target.xml support

This update allows GDB to query for the simulators's target description
(device architecture and register context format) and the simulator's
memory map.

I modified the MemorySim module to add a MemorySim_GetMemoryMapXML()
method which can build up the XML specific to how the memory is
currently configured.

I also had to grow the m_buffer member in mri4simBase test class since
the expected XML response stringe overflowed the previous 256 character
limit.