rfc: clobber all call-saved registers

[David Edelsohn]

>>>>> Richard Earnshaw writes:

Richard> Hmm, this might be useful as a way of safely allowing an asm to call a 
Richard> subroutine and ensure that the compiler clobbers all the correct registers 
Richard> at that point.

	Yes, this would be great!  I have needed the ability to use magic
instructions to trap into simulators that expect function-like argument
passing. 

David