X86JITInfo: [x86] Use X86CompilationCallback_SSE() along;

  *not* Subtarget->hasSSE1()
  *but* __SSE__, the flag that LLVM libraries are compiled

The callback calls internal LLVM JIT libraries. It may be built with -msse (or above).

FIXME: JIT may use "host" instead of "generic" by default.