Bump and fix experimental/firefox-esr

This MR bumps experimental/firefox-esr to 60.6.1 ESR (the latest release at this time), and updates the LLVM dependency to LLVM 7. Two new patches are required: the first fixes for building with Rust 1.33.0, as the build will fail due to undocumented macros, which I believe is a change in the behaviour of the deny_docs compiler pragma from 1.31.x; the second (courtesy of @aranea) fixes mozbuild to actually disable the profiling code when the --disable-profiling flag is passed in mozconfig, which otherwise fails to compile due to musl incompatibility.

Build succeeds on x86_64 with beta3 Rust and on aarch64 with beta2 Rust. Resulting package appears to work on both platforms, as my test boxen could both load https://adelielinux.org and play youtube videos.