package/icu: use the new ARCH_HAS_ATOMICS as dependency

And propagate to the reverse dependencies of icu.
Also, fix beecrypt's comment: only the C++ support needs atomics.

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>