kernel: Add CONFIG_SWAP_NONATOMIC flag

On ARM, _Swap() isn't atomic and a hardware interrupt can land after
the (irq_locked) caller has entered _Swap() but before the context
switch actually happens.  This will require some platform-specific
workarounds in a few places in the scheduler.

This commit is just the Kconfig and selection on ARM.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>