gcc: Fix riscv multilib issues in plain toolchains

gcc: Fix riscv multilib issues in plain toolchains

RISC-V multilib currently doesn't work at all. We could disable multilib for
the riscv platform but that would then behave differently to all our others.
Instead, copy the non-multilib config over the multilib config for now for
riscv, meaning we can keep the platforms similar.

This isn't quite enough as the triplet specific c++ headers are in the wrong
place leading to compiler issues and testimage failures. Work around that too
until someone adds full multilib support for the platform (if desired).

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>