Extract the code that records syscall results in the frame into MD

Extract the code that records syscall results in the frame into MD
function cpu_set_syscall_retval().

Suggested by:	marcel
Reviewed by:	marcel, davidxu
PowerPC, ARM, ia64 changes:	marcel
Sparc64 tested and reviewed by:	marius, also sunv reviewed
MIPS tested by:	gonzo
MFC after:	1 month