Update README.md for clear instructions on multilib

The current README.md only explains how to use multilib on the Linux cross-compiler, but not on the Newlib one. This might create confusion as seen here https://github.com/riscv/riscv-tools/issues/287, or makes it seem like you can't multilib on Newlib. It's also the case that the multilib option is more common on embedded toolchains rather than on Linux, so I believe this small change on documentation should clear things up.