arm: Refactor cortex-m4 irq_lock tmp handling.

Replace the use of a hardwired temporary register in the irq_lock()
implementation with a local variable.  This will allow the compiler
more flexibility in register allocation.

Change-Id: Ifbdb52fca1d40404d55934343ac2a8153df7e1a8
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>