Fix 'make TAGS' in gdbserver

PR build/29003 points out that "make TAGS" is broken in gdbserver.
This patch fixes the problem that is pointed out there, plus another
one I noticed while working on that -- namely that the "sed" computes
the wrong names for some source files.  Finally, a couple of obsolete
variable references are removed.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29003