powerpc/64: Add interrupt register sanitisation macros

Include in asm/ppc_asm.h macros to be used in multiple successive
patches to implement zeroising architected registers in interrupt
handlers. Registers will be sanitised in this fashion in future patches
to reduce the speculation influence of user-controlled register values.
These mitigations will be configurable through the
CONFIG_INTERRUPT_SANITIZE_REGISTERS Kconfig option.

Included are macros for conditionally zeroising registers and restoring
as required with the mitigation enabled. With the mitigation disabled,
non-volatiles must be restored on demand at separate locations to
those required by the mitigation.

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/20221201071019.1953023-2-rmclure@linux.ibm.com