bootstrap: Use hash to determine if sanitizers needs to be rebuilt
bootstrap: Use hash to determine if sanitizers needs to be rebuilt

* Rebuild sanitizers runtimes when LLVM submodule commit changes.
* When rebuilding LLVM / sanitizers, remove the stamp file before
  starting the build process to invalidate previous build output.

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust