Set up a self-hosted `musl` environment

`musl` is build-able now but largely unverified, it'll be great to set up a toolchain based on bootstrapped musl for more tests.
```
git clone --depth 1 git://git.musl-libc.org/musl
cd musl/
rm -rf src/complex/
AR=ar RANLIB=ranlib CC=~/slimcc/slimcc ../musl/configure --target=x86_64-linux-musl
```