ARC: atomics: Implement arch_atomic64_cmpxchg using _relaxed

The core atomic code has a number of macros where it elaborates
architecture primitives into more functions. ARC uses
arch_atomic64_cmpxchg() as it's architecture primitive which disable alot
of the additional functions.

Instead provide arch_cmpxchg64_relaxed() as the primitive and rely on the
core macros to create arch_cmpxchg64().

The macros will also provide other functions, for instance,
try_cmpxchg64_release(), giving a more complete implementation.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/Z0747n5bSep4_1VX@J2N7QTR9R3


Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>