Improved checking of target's llvm_config

Point llvm @bitshifter branch until PR accepted

Use today's date for LLVM auto clean trigger

Update LLVM submodule to point at rust-lang fork.

Handle case when target is set

src/rustllvm/llvm-auto-clean-trigger

@@ -1,4 +1,4 @@
 # If this file is modified, then llvm will be forcibly cleaned and then rebuilt.
 # The actual contents of this file do not matter, but to trigger a change on the
 # build bots then the contents should be changed so git updates the mtime.
-2016-07-25b
+2016-08-07