debug: gdbstub: add arch-specific funcs to read/write registers

This adds architecture-specific functions to read/write registers.
This allows architecture to have a sparse representation of
the register file as not all registers are saved during context
switches. This saves some runtime space, and provides some
flexibility on what architectures can do.

Remove from header the need to define ARCH_GDB_NUM_REGISTERS as
it is no longer used in the common gdbstub code.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>