Do not require SSE2 on i686
    
    See https://bugzilla.redhat.com/show_bug.cgi?id=1244196#c22 for the
    history of the patch. Compared to the last revision there (v5):
    * I fixed WebGLImageConversion.cpp to not assume SSE2 on x86.
      Unfortunately, it would be really hard to make runtime detection work
      there, because it uses intrinsics in inline templates, and GCC only
      supports intrinsics when building the whole code for the vector
      instruction set. So I had to turn off that SSE2 code entirely when
      __SSE2__ is not defined at compile time.
    * I fixed webrtc to check for SSE2 at runtime (using the runtime
      checking code that webrtc already has and uses in several places)
      before attempting to use OpenMAX on x86. OpenMAX requires SSE2.
      So we just fall back to the Ooura implementation of FFTs when needed.
    TODO: build V8 shared and twice (once for x87, once for SSE2)