powerpc: Save register r9-r13 values accurately on interrupt with bad stack

When we take an interrupt or exception from kernel mode and the stack
pointer is obviously not a kernel address (i.e. the top bit is 0), we
switch to an emergency stack, save register values and panic.  However,
on 64-bit server machines, we don't actually save the values of r9 - r13
at the time of the interrupt, but rather values corrupted by the
exception entry code for r12-r13, and nothing at all for r9-r11.

This fixes it by passing a pointer to the register save area in the paca
through to the bad_stack code in r3.  The register values are saved in
one of the paca register save areas (depending on which exception this
is).  Using the pointer in r3, the bad_stack code now retrieves the
saved values of r9 - r13 and stores them in the exception frame on the
emergency stack.  This also stores the normal exception frame marker
("regshere") in the exception frame.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>