[patch] Improve stack checking on PowerPC

[David Edelsohn]

	* config/rs6000/rs6000.md (UNSPEC_PROBE_STACK): New constant.
	(probe_stack): New insn.

Okay, but would you mind using the value "4" for the UNSPEC at the
beginning of the list?  I would like to fill in the empty values at
the beginning of the list and this instruction seems particularly
appropriate.

Thanks, David