gdbserver: remove support for M32R

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-m32r-low.cc.
	* configure.srv: Remove m32r case.
	* linux-m32r-low.cc: Remove.

Change-Id: I5617b2b1fd92aeec19b38e0e3c0b78adaafdb35b