rustup target add powerpc64le-unknown-linux-musl fails

[malbarbo]

With the message:

error: component 'rust-std' for 'powerpc64le-unknown-linux-musl' is unavailable for download

It is show in the output of rustup target list. It seems than powerpc64le-unknown-linux-musl is in the build manifest but is not built.

[pietroalbini]

powerpc64le-unknown-linux-musl is not present in our platform support list, so we don't guarantee official builds will be available, or that those builds will work.

cc @rust-lang/infra, we should probably clarify that the target is tier 3.

[malbarbo]

I think that is enough to remove the target from here.