Currently ARCH_HAVE_NMI_SAFE_CMPXCHG is not set on Arm systems and this
makes it impossible to enable features such as ftrace histogram triggers
on Arm platforms.

Most Arm systems are NMI safe simply because there is no NMI but this isn't
universally true meaning we cannot set ARCH_HAVE_NMI_SAFE_CMPXCHG for all
Arm devices. However the load/store exclusive implementation of cmpxchg is
NMI-safe and this implementation is used ARMv6k and later. Let's select
ARCH_HAVE_NMI_SAFE_CMPXCHG for these systems.

Note that ARMv6 uses load/store exclusive for 32-bit cmpxchg but relies on
interrupt masking for 8- and 16-bit operations. This patch is conservative
and does not change behaviour for CPU_V6.
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>