ARM: ptrace: Add APSR_MASK definition to ptrace.h

APSR_MASK can be used to extract the APSR bits from the CPSR. The
comment for these definitions is also changed because it was inaccurate
as the existing defines didn't refer to any part of the APSR.

Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>