Musl is great for embedded projects since it doesn't have any
external runtime dependencies on static builds (glibc needs
libnss for gethostbyname etc), and produces much smaller binaries.
It's also needed for supporting Alpine Linux (the most common distro
used for containers).

This patch expects the latest riscv-musl repository to be cloned
inside the source dir (e.g. as a submodule like the rest). Currently
only riscv64 is supported (no multilib).