Well, adding some stuff to the source's .cargo/config allowed me to compile:

$ cat ../../.cargo/config [build] rustflags = ["-C", "link-args=-ldl -lutil -lpthread"]

The result is

target/mipsel-unknown-linux-musl/release/sdl-demo: ELF 32-bit LSB shared object, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/ld.so.1, for GNU/Linux 3.2.0, BuildID[sha1]=36239a5af12cc0039a0725446fc473b1dc16ff1c, with debug_info, not stripped

But the resulting binary does not seem to work