add memory barrier to arch_local_irq_restore

arch_local_irq_save() and friends are required to act as compiler
memory barriers. This patch adds a "memory" clobber to the inline
asm code in arch_local_irq_restore() which is used as the building
block for other functions needing to set/clear the interrupt enable
in the CSR register.

Signed-off-by: Mark Salter <msalter@redhat.com>