[Bug libffi/84410] libffi doesn't support riscv now, but not disabled in configure.ac

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84410

--- Comment #2 from Jim Wilson <wilson at gcc dot gnu.org> ---
FYI There is a libffi port at
    https://github.com/riscv/riscv-libffi
but it is for an old ABI.  There are comments in issue 1 pointing at more up to
date but still not complete ports.