This rewrites the PPC64 exception entry/exit routines to make them
smaller and faster.

In particular we no longer save all of the registers for the common
exceptions - system calls, hardware interrupts and decrementer (timer)
interrupts - only the volatile registers.  The other registers are saved
and restored (if used) by the C functions we call.  This involved
changing the registers we use in early exception processing from r20-r23
to r9-r12, which ended up changing quite a lot of code in head.S. 
Overall this gives us about a 20% reduction in null syscall time. 

Some system calls need all the registers (e.g.  fork/clone/vfork and
[rt_]sigsuspend).  For these the syscall dispatch code calls a stub that
saves the nonvolatile registers before calling the real handler.

This also implements the force_successful_syscall_return() thing for
ppc64.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>