Follow-on to SERVER-12064. Robie Basak of Canonical submitted a pull request to improve our portability, particularly on ARM.

https://github.com/mongodb/mongo/pull/582

We merged two of his three commits for 2.5.5. The third changes the codepath on IA-32 and x86_64 to use GCC atomic builtins if available rather than the inline assembler of mongo/platform/atomic_intrinsics_gcc_intel.h