Add a debugging interface for PowerPC that allows signal handlers (or any
jump to a context, really) to perform debug functions.  It allows the a
user program 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.

Note that you can enable single-stepping on x86 processors directly from
signal handlers.  Newer x86 processors have the equivalent of a
branch-trace bit in the IA32_DEBUGCTL MSR and could have similar function
to this syscall.  Most other processors could benefit from a similar
interface, except for ARM which is extraordinarily broken for debugging.

Future uses of this could be adding the ability to set the hardware
breakpoint registers from a signal handler.
Signed-off-by: Corey Minyard <minyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>