powerpc: Remove use of a second scratch SPRG in STAB code

The STAB code used on Power3 and RS/64 uses a second scratch SPRG to
save a GPR in order to decide whether to go to do_stab_bolted_* or
to handle a normal data access exception.

This prevents our scheme of freeing SPRG3 which is user visible for
user uses since we cannot use SPRG0 which, on RS/64, seems to be
read-only for supervisor mode (like POWER4).

This reworks the STAB exception entry to use the PACA as temporary
storage instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>