Disable gdbserver on host != target configurations

Disable gdbserver on host != target configurations

Correct fallout from commit 919adfe84092 ("Move gdbserver to top level")
and revert to not building `gdbserver' in a cross-configuration, that is
where host != target, matching the documented behaviour.  We have no way
to support non-native `gdbserver', and native `gdbserver' is usually of
no use with cross-GDB of the chosen host.

gdbserver/ChangeLog:
2020-02-12  Maciej W. Rozycki <macro@wdc.com>
	    Pedro Alves  <palves@redhat.com>

	Skip building gdbserver in a cross-configuration.
	* configure.srv: Set $gdbserver_host depending on whether $target
	is $host.  Use $gdbserver_host instead of $host.