add preprocessor hooks for C11, GCC, BGQ and PPC atomics

GCC is in-progress.  C11 is yet to be created.  BGQ will use PPC for
now.  PPC is in-progress.

BGQ exists beyond PPC in case we want to use L2 atomics or any other
fancy HW features or we find PPC atomics need e.g. more mbars than
expected.
GCC atomics implementation

this needs to have unit tests for verification.