powerpc: avoid -mno-sched-epilog on GCC 4.9 and newer

commit 6977f95e upstream.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[nc: Adjust context due to lack of f2910f0e and 2a056f58]
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>