arm64: Remove redundant mov from LL/SC cmpxchg

The cmpxchg implementation introduced by commit c342f782 ("arm64:
cmpxchg: patch in lse instructions when supported by the CPU") performs
an apparently redundant register move of [old] to [oldval] in the
success case - it always uses the same register width as [oldval] was
originally loaded with, and is only executed when [old] and [oldval] are
known to be equal anyway.

The only effect it seemingly does have is to take up a surprising amount
of space in the kernel text, as removing it reveals:

   text	   data	    bss	    dec	    hex	filename
12426658	1348614	4499749	18275021	116dacd	vmlinux.o.new
12429238	1348614	4499749	18277601	116e4e1	vmlinux.o.old

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>