C++11 atomics using gcc and clang intrinsics

C++11 atomics using gcc and clang intrinsics

For environments using LLVM 3.1 or GCC 4.7 or better, use gcc and clang intrinsics and inline bsls atomics.
Notably, bsls atomics support for ARM is included when the toolchain is LLVM 3.1 or GCC 4.7 or better.