Add preprocessor defines when SSE is enabled

This will add the following preprocessor defines when Haskell source
files are compiled:

 * __SSE__ - If any version of SSE is enabled
 * __SSE2__ - If SSE2 or greater is enabled
 * __SSE4_2_ - If SSE4.2 is enabled

Note that SSE2 is enabled by default on x86-64.
Merge branch 'master' of https://github.com/ghc/ghc