target/m68k: use gdb_get_reg helpers

target/m68k: use gdb_get_reg helpers

This is cleaner than poking memory directly and will make later
clean-ups easier.

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20200316172155[email protected]>