Right now, there are no prebuilt packages for Rust on ppc-musl and ppc64-musl, and bootstrap on the host machines (ppc or ppc64) cannot be done, because the Rust project doesn't publish the necessary prerequisite binary stage0s.
These targets are, however, supported by Rust, and it is possible to build binary packages of rustc by cross-compiling from a build system with a working Rust installation. After such bootstrap, Rust can be