Can you try building your crate on Adélie Linux?  We've specifically
enabled dynamic linking in rustc for musl libc, which should provide you
these features.

Let me know.  If it does work, then this should probably be noted in one
of the issue trackers for Rust.

Additionally, our 'gcompat' package should provide both of your missing
symbol:

libgcompat/pthread.c:28:int __register_atfork

__res_init should be provided by musl, I thought, but we can add an
alias if needed.

Best,
--arw

-- 
A. Wilcox (awilfox)
Project Lead, Adélie Linux
https://www.adelielinux.org