ARM: 6889/1: futex: add SMP futex support when !CPU_USE_DOMAINS

This patch uses the load/store exclusive instructions to add SMP futex
support for ARM.

Since the ARM architecture does not provide instructions for
unprivileged exclusive memory accesses, we can only provide SMP futexes
when CPU domain support is disabled.

Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>