nanokernel: support GCC compiler atomic builtins

Arches now select whether they want to use the GCC built-ins,
their own assembly implementation, or the generic C code.

At the moment, the SDK compilers only support builtins for ARM
and X86. ZEP-557 opened to investigate further.

Change-Id: I53e411b4967d87f737338379bd482bd653f19422
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>