Not sure if something like this should be part of glibc. Just recently there was an ABI break due to the fact that jmp_buf is exposed in the headers to allow embedding the struct[1].

[1] https://lwn.net/Articles/605607/

(Also, I ended up mixing up ucontext.h with setjmp.h in my comment above; Lwan uses ucontext.h as a fallback. There are coroutine implementations that will use setjmp/longjmp, or at the very least reuse the jmp_buf struct and roll their own asm, though.)