Waking up this thread, as I made some progress.

I finally build a PCC toolchain using musl as its libc. It's not entirely GNU Free though, as it still need GNU binutils to work. But it's a huge step forward!
I've been looking into elftoolchain (from the fedora project) and elfutils (freebsd iirc) but none of them include an assembler, and only elftoolchain provides a linker (not sure if it's working though).

Regarding the PCC+musl toolchain, I blogged about it, you can find everything about it here: http://blog.z3bra.org/2015/08/cross-comp...-musl.html
If you have any questions about it, or think some infos are missing or not crystal clear, tell me and I'll rework this article.