gdbserver: Make `make TAGS' actually work

Fix a:

make: *** No rule to make target '.../gdb/gdbserver/arch/arm.c', needed by 'TAGS'.  Stop.

error produced by `make TAGS' by making the list of sources processed
match actual file locations and by moving host-specific object files
listed in DEPFILES to nat/ or target/ subdirectories as appropriate so
that the location of the corresponding source file can be mechanically
determined.

	gdb/gdbserver/
	* Makefile.in (SFILES): Adjust paths to point to real files.
	(OBS): Move waitstatus.o to target/waitstatus.o.
	(TAGS): Transform paths appropriately.
	(%.o): Rename to...
	(nat/%.o): ... this pattern rule.
	(%.o): Rename to...
	(target/%.o): ... this pattern rule.
	* configure.srv: Adjust paths throughout to include nat/ prefix
	with the revant files.
	* configure.ac: Add `nat' and `target' to CONFIG_SRC_SUBDIR.
	* configure: Regenerate.