mk: add mechanisms for triggering clean-llvm builds from commits
auto merge of #7442 : graydon/rust/clean-llvm-trigger, r=brson

This makes it possible for us to trigger the llvm-clean make-target by checking in a change that touches rustllvm/llvm-auto-clean-stamp. Most developers don't need to see or know about this, but when you push a change that "needs an LLVM rebuild", even if not otherwise obvious, this should give a mechanism to do it.