arch/arc: fix atomics selection

Due to a kconfig limitation, we can't select a no-prompt symbol that
gets its dependencies by being conditionally re-defined in one or more
if-blocks, like we currently do for BR2_ARCH_HAS_ATOMICS.

As a workaround to this issue, we just redefine that symbol in the arc
if-block, like we do for all other architectures, except that in the arc
case, the default value is conditional.

Reported-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
[yann.morin.1998@free.fr: indepently re-done a patch similar to the one
 Thomas made on his own]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>