powerpc: make ARCH=ppc use arch/powerpc/kernel/process.c

Commit 5388fb10 made signal_32.c
use discard_lazy_cpu_state, which broke ARCH=ppc because that
uses the common signal_32.c but has its own process.c.  Make ARCH=ppc
use the common process.c to fix this and to reduce the amount
of duplicated code.

Signed-off-by: Paul Mackerras <paulus@samba.org>