Update HFILES_NO_SRCDIR in gdb/Makefile.in

I noticed a header file from dwarf2/ was missing from HFILES_NO_SRCDIR
in gdb/Makefile.in.  Looking more, I found many missing files.  This
patch adds them all and sorts the list -- using "sort", though, and
not the advice at the top of Makefile.in that, IMO, seems hard to
implement.

This also removes some code from the 'tags' rule that I think is
obsolete.