powerpc/64s: IOption for MSR stored in r12

Interrupt handlers in asm/exceptions-64s.S contain a great deal of common
code produced by the GEN_COMMON macros. Currently, at the exit point of
the macro, r12 will contain the contents of the MSR. A future patch will
cause these macros to zeroise architected registers to avoid potential
speculation influence of user data.

Provide an IOption that signals that r12 must be retained, as the
interrupt handler assumes it to hold the contents of the MSR.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221201071019.1953023-4-rmclure@linux.ibm.com