Multilib support

[thesamesam]

When trying to wire up rustc_codegen_gcc in our packaging, I hit the following when trying to build multilib on x86_64 for i686 (i686-unknown-linux-gnu):

libgccjit.so: error: CPU you selected does not support x86-64 instruction set

This is because the GCC built by dist passes --disable-multilib. But I suspect more wiring may needed too. I see gcc/jit/jit-playback.cc:3514:add_multilib_driver_arguments at least, so libgccjit upstream seems to have some support for it.