C6X: select GENERIC_ATOMIC64

The generic atomic64 support came in 2009 to support the perf subsystem
with the expectation that all architectures would implement atomic64
support. Since then, other optional parts of the generic kernel have
also come to expect atomic64 support. This patch enables generic atomic64
support for C6X architecture.

Signed-off-by: Mark Salter <msalter@redhat.com>