restore -target wasm32-freestanding-musl for C headers
See #5854
Some tiny tweaks too:
* Use `wasm-freestanding-musl` instead of `wasm32-freestanding-musl`,
making it pointer-size-agnostic.
* Fix trying to build non-existent wasm musl start files.
d2445764a9Andrew Kelley committed on 12/1/2020, 3:58:31 AMparenta9b7d8f