Backport multilib selection

Created by: simonpcook

This back ports the addition of converting multilib selection to a target hook, for which a RISC-V specific implementation is then added to choose the most appropriate multilib. As an example:

$ ./riscv32-corev-elf-gcc -print-multi-directory -mabi=ilp32e -march=rv32e_zba_zbb

returns

rv32e/ilp32e

rather than the default multilib

(This should just be a fast forward of the existing development branch, this message shouldn't end up in the final repo)