Merge top-level configury changes from gdb

Merge top-level configury changes from gdb

We recently rearranged the gdb source tree to move a common library
and gdbserver to the top-level.  This made the build more uniform and
also a bit faster (due to sharing of built objects).

This patch re-syncs these changes the top-level configury back to gcc.

ChangeLog:
	* configure: Rebuild.
	* Makefile.in: Rebuild.
	* Makefile.def (gdbsupport, gdbserver): New host modules.
	(configure-gdb): Depend on all-gdbsupport.
	(all-gdb): Depend on all-gdbsupport, all-libctf.
	* configure.ac (host_tools): Add gdbserver.
	Conditionally build gdbserver and gdbsupport.