powerpc/32: Replace ASM exception exit by C exception exit from ppc64

This patch replaces the PPC32 ASM exception exit by C exception exit.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/48f8bae91da899d8e73fc0d75c9af66cc97b4d5b.1615552867.git.christophe.leroy@csgroup.eu