Change gdbserver to use existing gdbsupport

This changes the gdbserver build to use the gdbsupport that was built
for gdb.

gdbserver and gdbreplay now must use WIN32APILIBS (aka -lws2_32).
Before this change, gdbserver did not define USE_WIN32API when
building gdbsupport, but now this is always done.

ChangeLog
2020-03-12  Tom Tromey  <tom@tromey.com>

	* Makefile.in: Rebuild.
	* Makefile.def (gdbserver): Depend on gdbsupport.

gdbserver/ChangeLog
2020-03-12  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* configure.ac (GDBSERVER_DEPFILES): Remove srv_selftest_objs.
	(WIN32APILIBS): New subst.
	* Makefile.in (SFILES, OBS, TAGS, GDBREPLAY_OBS): Remove
	gdbsupport files.
	(gdbsupport/%.o): Remove target.
	(GDBSUPPORT_BUILDDIR, GDBSUPPORT): New variables.
	(gdbserver$(EXEEXT), gdbreplay$(EXEEXT)): Add GDBSUPPORT.
	(WIN32APILIBS): New variable.
	(gdbserver$(EXEEXT)): Add WIN32APILIBS.
	(gdbreplay$(EXEEXT)): Likewise.