Initialize the target architecture based on compiler defines, so if compiled on

Initialize the target architecture based on compiler defines, so if compiled on
x86 or Sparc, LLC will automatically default to that platform, no guessing
required. On another platform, it will default to `noarch' and will have to
guess which architecture to compile to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7207 91177308-0d34-0410-b5e6-96231b3b80d8