Exception return implies context synchronization, so we can hook up the
SYNC_CORE option to sys_membarrier() simply by selecting the Kconfig option,
just like we've done for arm64 already.

Cc: Orion Hodson <oth@google.com>
Acked-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>