ARM: Avoid writing to control register on every exception

If we are not changing the control register value, avoid writing to it.
Writes to the control register can be very expensive, taking around a
hundred cycles or so.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --cc arch/arm/kernel/entry-armv.S
Simple merge

diff --cc arch/arm/kernel/entry-common.S
Simple merge

diff --cc arch/arm/kernel/entry-header.S
Simple merge