kernel: Fix CC clobber in cortex_m4 irq_lock()

The cortex-m4 irq_lock() implementation uses a movs instruction, thus
clobbering the condition code, but does not include the clobber in the
asm clobber list.  This is a bug in the situation where the compiler
schedules a live condition code over the inline lock instructions.

Since the irq_lock() implementation does not need to kill the CC we
simply switch to mov from movs.

Take the opportunity to drop the unnecessay .n and let the assembler
choose an appropriate encoding.

This fixes a bug found by inspection, it has not actually been
observed in real code.

Change-Id: Id60fa3362df9d4bf05c3d5e23066410ede92d73c
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>