"make TAGS" broken in 7.8.90

[Eli Zaretskii <eliz@gnu.org>]

"make TAGS" is broken.  Again.  (Sergio, would it make sense to add
"make TAGS" to the list of commands run by the buildbot after a build?
This breaks too often, IMO.)

     Doing TAGS in gdb
     make[2]: Entering directory `/d/usr/eli/utils/gdb-7.8.90/gdb'
     make[2]: *** No rule to make target `ada-varobj.h', needed by `TAGS'.
     make[2]: Target `TAGS' not remade because of errors.
     ./x86-dregs.c: No such file or directory
     ./irix5-nat.c: No such file or directory

ada-varobj.h and irix5-nat.c should simply be removed from the
respective lists.  But I'm not sure how to fix the x86-dregs.c issue.
The problem here is that x86-dregs.o is in gdb/, but the source is in
gdb/nat/, so the simplistic editing:

	done) | sed 's/\.o$$/\.c/'` \

doesn't work.  Thoughts?

One other annoyance with "make TAGS" is that libdecnumber doesn't
support that target, so you must invoke the command from gdb/, or use
"make -k", otherwise it bails out.  Any reason not to support TAGS in
libdecnumber?