Add a mostlyclean target to gdbserver

I noticed gdbserver's Makefile doesn't have a mostlyclean target.  I
use this sometimes, so this patch adds it.  Adding it as a synonym of
"clean" seemed to make sense, given the contents of that rule.

gdbserver/ChangeLog
2020-02-21  Tom Tromey  <tom@tromey.com>

	* Makefile.in (mostlyclean): New target.