Have testsuite find gdbserver in new location

Have testsuite find gdbserver in new location

This updates the gdb testsuite to look for gdbserver in its new
location.  The old location is also checked for, on the theory that
perhaps someone sets GDB to a full path for install testing.

gdb/testsuite/ChangeLog
2020-02-14  Tom Tromey  <tom@tromey.com>

	* lib/gdbserver-support.exp (find_gdbserver): Find gdbserver in
	build directory.
	* boards/gdbserver-base.exp: Update path to gdbserver.

Change-Id: If03db762ba53882ddfaf2d2d516de14c3fa03938