libdebugger: Add accessor for cpu state size.

Architecture:
- Store and provide accessor for the size in bytes of the low-level
debug_cpu_state size of the respective target CPU. Adjust subclasses
to pass in the appropriate size information.

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.