llvm2.9: add LLVM_EXTRA_ARCH to enable X86 unless building for x86

* now I see why it was there before, at least on arm it fails with:
  | CMake Error at cmake/modules/LLVMConfig.cmake:127 (message):
  |   Library `X86' not found in list of llvm libraries.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>