Make stackmap machineinstrs clobber the scratch regs too.

Patchpoints already did this. Doing it for stackmaps is a convenience
for the runtime in the event that it needs to scratch register to
patch or perform a runtime call thunk.

Unlike patchpoints, we just assume the AnyRegCC calling
convention. This is the only language and target independent calling
convention specific to stackmaps so makes sense.  Although the calling
convention is not currently used to select the scratch registers.