Rollup merge of #55562 - smaeul:powerpc-linux-musl, r=alexcrichton

Add powerpc- and powerpc64-unknown-linux-musl targets

Add targets for musl on 32-bit and 64-bit powerpc. This requires some ABI fixes, as musl [uses the ELFv2 ABI on regardless of endianness](http://git.musl-libc.org/cgit/musl/tree/configure?id=8084d6ab57cdb0b8f328d3cdbad3b9d09eaaee04#n638). At the moment, powerpc64 support requires [an LLVM patch](https://reviews.llvm.org/D52013) to select the correct ABI; or I can add [a patch to Rust's LLVM backend](https://github.com/smaeul/rust/commit/e8eaa2afd51ed9950096359a040ba1603907cdd1) to always choose the right ABI.

Both architectures are able to run an extended bootstrap, and with some test fixes (e.g. #55561), there are no architecture-dependent test failures on powerpc64 (most failures in `src/test` are existing musl-host-related issues).