Auto merge of #36292 - japaric:musl-root, r=alexcrichton
rustbuild: per target musl-root
config.toml now accepts a target.$TARGET.musl-root key that lets you
override the "build" musl-root value, which is set via the --musl-root
flag or via the build.musl-root key.
With this change, it's now possible to compile std for several musl
targets at once. Here's are the sample commands to do such thing: