Add AIX/POWER atomicops and fix compilation with IBM xlC C++ compiler.

These patches adds initial support for POWER 32 and 64-bit atomic operations and fixes up various little things that lets the project and tests compile cleanly with the IBM xlC C++ compiler on AIX.
I put our name here because I saw that `atomicops_internals_generic_gcc.h` contained copyright by Red Hat. I can always change it -- just wanted to make sure I was doing the proper thing.