Build gdb "nat" files in subdirectory

Build gdb "nat" files in subdirectory

This moves the various "nat" object files into the nat/ subdirectory.
This allows for the removal of a pattern rule from the gdb Makefile,
which is a small cleanup.

I made the configure.nat change in a (semi-) automated way, hopefully
meaning that it is more likely to be correct than had I done it by
hand.

Eventually I would like for the various configure scripts to only
mention source files, and let the Makefile compute the object file
names.

gdb/ChangeLog
2018-12-27  Tom Tromey  <tom@tromey.com>

	* configure.nat (NATDEPFILES): Use nat/ prefix.
	* Makefile.in (CONFIG_SRC_SUBDIR): Add nat.
	(%.o: ${srcdir}/nat/%.c): Remove rule.
	(INIT_FILES): Do not filter out NATDEPFILES.