rs6000: rs6000_dbx_register_number for fp/ap/mq

rs6000: rs6000_dbx_register_number for fp/ap/mq

The frame pointer and the argument pointer aren't real registers.  MQ
was a register on old POWER.  All three are still used as arguments to
rs6000_dbx_register_number during initialisation.  If we handle them
explicitly we can do a gcc_unreachable to catch other unexpected
registers.

	* config/rs6000/rs6000.c (rs6000_dbx_register_number): Handle
	FRAME_POINTER_REGNUM, ARG_POINTER_REGNUM, and 64 (which was MQ).

From-SVN: r270925