debug: gdbstub: allow aligned memory access

Some architectures may require memory accessed to be aligned to
certain size and cannot be accessed byte-by-byte during memory
read/write in GDB stub. This adds the ability to specify
the alignment via kconfig. The existing byte-by-byte access is
retained as it is simplier code.

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