gdbserver: remove support for CRIS

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): Remove linux-cris-low.c.
	* configure.srv: Remove cris cases.
	* linux-cris-low.cc, linux-crisv32-low.cc: Remove.

Change-Id: Ib3ff436b03373548215f15540a47f39cbec5f512