emsdk

Optimize sandbox performance (#1045)

* Optimize sandbox performance

Link just the files needed to compile, link, or archive, rather than the entire directory tree. This drastically improves build times on macOS, where the sandbox is known to be slow (https://github.com/bazelbuild/bazel/issues/8230).

* Linux wants clang to link

* all_files not needed?

* Linux wants llc

* And llvm-ar

* Templated build_file_content