mk: Fix llvmdeps.rs dependencies

In a multi-host build the mklldeps.py tool is getting called before
all the llvm-configs are built. I am not actually sure the cause. I
had convinced myself that DEF_LLVM_RULES needed to be called before
the llvmdeps.rs rule, but now looking at it again I can't see why.
auto merge of #11292 : brson/rust/llvmconfig, r=alexcrichton

In a multi-host build the mklldeps.py tool is getting called before
all the llvm-configs are built. I am not actually sure the cause. I
had convinced myself that DEF_LLVM_RULES needed to be called before
the llvmdeps.rs rule, but now looking at it again I can't see why.

Here's the error: http://buildbot.rust-lang.org/builders/dist2-linux/builds/11/steps/compile/logs/stdio

r? @alexcrichton