Add initial support for multilibs in Baremetal toolchain.

This patch add support of riscv multilibs in the Baremetal toolchain. It is
a bit different to what is done in GNU.cpp as we are not iterating a
GNU sysroot to find the multilibs. This is intended for an llvm only
toolchain. We are not checking for the presence of any runtime bits to
enable a specific multilib.

I have structured the patch so that other targets for which
there is no multilibs support yet in Baremetal.cpp (e.g. arm-none-eabi)
will not be affected. Patch also allows some multilibs reuse.

Long term, I would like to go in the direction of data-driven specification of
multilib directories and flags.

Reviewed By: jroelofs

Differential Revision: https://reviews.llvm.org/D93138