ARM: 6213/1: atomic64_test: add ARM as supported architecture

ARM has support for the atomic64_dec_if_positive operation
so ensure that it is tested by the atomic64_test routine.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>