powerpc: mark more local variables as volatile

A while ago I created a2305e3d ("powerpc: mark local variables
around longjmp as volatile") in order to allow building powerpc with
-Wextra enabled on gcc-11.

I tried this again with gcc-13 and found two more of the same issues,
presumably based on slightly different optimization paths being taken
here:

arch/powerpc/xmon/xmon.c:3306:27: error: variable 'mm' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
arch/powerpc/kexec/crash.c:353:22: error: variable 'i' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]

I checked a bunch of randconfigs and found only these two, so just
address them the same way as the others.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230809131024.2039647-1-arnd@kernel.org