[Bug target/84797] RISC-V: add --with-multilib-list support

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84797

--- Comment #1 from Jim Wilson <wilson at gcc dot gnu.org> ---
Created attachment 43610
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43610&action=edit
Initial proposed patch.

Currently, this only supports rv64gc arch, and only allows ABIs in
--with-multilib-list, as handling every arch/abi combination gets painful
pretty quickly.  We can always extend this later if necessary.

I haven't added multiarch support yet, but that should be pretty simple if
necessary once we have agreed on the dir names to use for that.