gdb/MAINTAINERS: restore m68hc11, score and xstormy16 entries

This patch restores some entries removed by a recent patch whose purpose
was to update the list of active maintainers. I thought that, the target
information was purely to document the scope of the given target, and
thus could be removed is maintainerless. But, in fact, those entries
are still useful, as a number of scripts (eg: gdb_buildall.sh) use
that information to build GDB with all targets enabled.

gdb/ChangeLog:

	* MAINTAINERS: Restore target entries for m68hc11-elf,
	score-elf and xstormy16-elf, incorrectly removed in a previous
	patch meant to only update the list of active maintainers.