Russ implemented his own setmcontext and getmcontext functions
to work on systems that doesn't properly support ucontext.
So I don't think you really need ucontext support in your libc.
By the way, I maintain an updated version of libtask:
https://github.com/0intro/libtask
I've used it on quite a few places over the years,
but I only on POSIX systems so far.
--
David du Colombier