arch: add an option to specify if the arch has atomic ops

The fact that atomic operations are available is not really a
specificity of the toolchain, but rather of the architecture.

So, add a new option that architectures that have atomic operations
can select. This in turn selects the current toolchain atomic option,
until all packages have been converted, at which point the old
toolchain option can be removed.

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