Rather than relying on the system to have cross toolchains available,
simply download musl.cc's ones and use that libc.so, and then we use it
to fill in a few missing platforms, such as riscv64, riscv64, powerpc64,
and s390x.

Since riscv doesn't have a second serial port in its device description,
we have to use virtio's vport. This is actually the same situation on
ARM, but we were previously hacking QEMU up to work around this, which
required a custom QEMU. Instead just do the vport trick on ARM too.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>