Return Maybe Platform as part of compiler configure, and place it in...

Return Maybe Platform as part of compiler configure, and place it in LocalBuildInfo as hostPlatform.
GHC infers the platform form ghc --info using new 'platformFromTriple' function. Other compilers return Nothing, which triggers fallback to old behavior of using buildPlatform. hostPlatform is then threaded through to initialPathTemplateEnv.