ARM: 7895/1: signal: fix armv7-m build issue in sigreturn_codes.S

After "ARM: signal: sigreturn_codes should be endian neutral to
work in BE8" commit, thumb only platforms, like armv7m, fails to
compile sigreturn_codes.S. The reason is that for such arch
values '.arm' directive and arm opcodes are not allowed.

Fix conditionally enables arm opcodes only if no CONFIG_CPU_THUMBONLY
defined and it uses .org instructions to keep sigreturn_codes
layout.

Suggested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>
Tested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>