Add handling for 3900's SDBBP, DERET, and RFE insns.

Add handling for 3900's SDBBP, DERET, and RFE insns.

	* gencode.c (SDBBP,DERET): Added (3900) insns.
	(RFE): Turn on for 3900.
	* interp.c (DebugBreakPoint,DEPC,Debug,Debug_*): Added.
	(dsstate): Made global.
	(SUBTARGET_R3900): Added.
	(CANCELDELAYSLOT): New.
	(SignalException): Ignore SystemCall rather than ignore and
	terminate.  Add DebugBreakPoint handling.
	(decode_coproc): New insns RFE, DERET; and new registers Debug
	and DEPC protected by SUBTARGET_R3900.
	(sim_engine_run): Use CANCELDELAYSLOT rather than clearing
	bits explicitly.
	* Makefile.in,configure.in: Add mips subtarget option.
	* configure: Update.