scons: Generate SSE2 floating-point arithmetic.

scons: Generate SSE2 floating-point arithmetic.

- SSE2 is available on all x86 processors we care about.

- It's recommended by Intel:

  https://software.intel.com/en-us/blogs/2012/09/26/gcc-x86-performance-hints

- And has been the default since MSVC 2012:

  http://msdn.microsoft.com/en-us/library/7t5yh4fd(v=vs.110).aspx

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>