gdbserver: remove support for Tile

This port has been unmaintained for years and the upstream Linux kernel
does not support this architecture anymore, remove it.

gdbserver/ChangeLog:

	* Makefile.in (SFILES): linux-tile-low.cc.
	* configure.srv: Remove tilegx case.
	* linux-tile-low.cc: Remove.

Change-Id: I1c2910d04ddbd6013e5d228047106b41d80f9477