Rollup merge of #36972 - nastevens:fix-rustbuild-per-target-musl-root, r=alexcrichton

Fix rustbuild per-target musl root

In #36292, support was added to target musl libc for ARM targets using rustbuild. Specifically, that change allowed the addition of per-target `musl-root` options in the rustbuild `config.toml` so that multiple targets depending on musl could be built. However, that implementation contained a couple of omissions: the `musl-root` option was added to the config, but was never added to the TOML parsing, and therefore was not actually being loaded from `config.toml`. This PR rectifies that.

Using these changes and a heavily modified version of the buildbot Docker container, I have been able to build rust targeting `armv7-unknown-linux-musleabihf` and have successfully run the binaries on a Raspberry Pi 3. I'm also planning to test `arm-unknown-linux-musleabi` and `arm-unknown-linux-musleabihf` systems, but have no reason to believe that this change would not simply work on those targets.