Bug 1025393 - Enable building webrtc with clang-cl; r=jesup

--HG--
extra : rebase_source : 16c3846d3a31b71e4ba3f9e4214c1ef8ff6a03e4
Bug 1025900 - #include intrin.h directly in Atomics.h instead of declaring functions manually from it; r=froydnj

This is basically more of bug 1022049.

--HG--
extra : rebase_source : 39cf2745d47d90ba1684c2c26a38f9781a4d179e