arm64: Make GENERIC_ATOMIC64 implementation compile for arm64

GENERIC_ATOMIC64 is a C implementation of the atomic64_xxx APIs.
Enabling this feature requires some modifications in the generic
header files in order to make this feature compile for the arm64
targets.

Change-Id: Id7e55bd355b61e6b29a6622b353045b735fb011e
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>