[AVR32] Add TIF_RESTORE_SIGMASK to the work masks

We really need to check TIF_RESTORE_SIGMASK before returning to
userspace. The existing code does not necessarily do this.

Define the work masks as a bitwise OR of the respective flags instead
of a hardcoded hex value to make it easier to spot errors like this in
the future.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>