powerpc/interrupt: Do not call single_step_exception() from other exceptions

single_step_exception() is called by emulate_single_step() which
is called from (at least) alignment exception() handler and
program_check_exception() handler.

Redefine it as a regular __single_step_exception() which is called
by both single_step_exception() handler and emulate_single_step()
function.

Fixes: 3a96570f ("powerpc: convert interrupt handlers to use wrappers")
Cc: stable@vger.kernel.org # v5.12+
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aed174f5cbc06f2cf95233c071d8aac948e46043.1628611921.git.christophe.leroy@csgroup.eu