I'm using MacPorts and I have no idea how to hand compile just libgccjit there. The libgcc port doesn't even produce the jit language.
port install emacs-devel +nativecomp
port install emacs-app-devel +nativecomp
You may try to report a bug[0] for Mojave but I wouldn't hold my breath.
[0] https://trac.macports.org/wiki/MojaveProblems
Macports has gccjit since gcc10, just somebody needs to update Emacs.
What others archs? All of course
I'm writing my own Portfile to get libgccjit 11 now. The emacs-devel port does bring in the entire gcc10, but I just want libgccjit.
I'm using MacPorts and I have no idea how to hand compile just libgccjit there. The libgcc port doesn't even produce the jit language.