arch/arm: always has atomic ops

armv6 and above all have one sort of atomic ops or another. For armv5
and below, they are emulated, either as a kernel trap, a kernel VDSO,
or compiler intrinsics.

Aarch64 is just armv8, so make it a single commit. ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Anton Kolesov <Anton.Kolesov@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>