Add MicroBlaze support for libdebugger. This uses only software break
type instructions to provide self-hosted GDB debugging support for
applications since internal control of debug hardware is not possible.
Also of note, this implementation for MicroBlaze would typically use the
brki instruction for software break, but instead uses an illegal opcode
to manage software breaks as exceptions. This is due to poor interaction
with the debug hardware where the debug hardware will intercept software
breaks instead of allowing the software break vector to execute.
