PPC debug setcontext syscall implementation

Hello all,

I have attached a patch with the implementation of the debug_setcontext 
system call.  The syscall has been reserved for a while and I've posted 
this before.  So I've ported to the newest kernel and here it is again.

This syscall allows signal handlers to perform debug functions.  It 
allows the signal handler to turn on single-stepping, for instance, and 
the thread will get a trap after executing the next instruction.  It can 
also (on supported PPC processors) turn on branch tracing and get a trap 
after the next branch instruction is executed.  This is useful for 
in-application debugging.

A small test program is also attached that demonstrates the use of this 
interface.

Thanks,

-Corey