Summary:
[Folly] Apply `clang-format` to `folly/portability/`.

With some manual rearrangement in the places where `clang-format` does awkward things, with the result that `clang-format` over `folly/portability/` becomes a no-op.

Reviewed By: igorsugak

Differential Revision: D5170107

fbshipit-source-id: ceadd96740b4877cbae947846f0e738e6aaf5ed1