[RISCV] Add driver for riscv32-unknown-elf baremetal target

Summary:
This patch adds a driver for the baremetal RISC-V target (i.e. riscv32-unknown-elf). For reference, D39963 added basic target info and added support for riscv32-linux-unknown-elf.

Patch by: asb (Alex Bradbury)

Reviewers: efriedma, phosek, apazos, espindola, mgrang

Reviewed By: mgrang

Subscribers: jrtc27, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, emaste, mgorny, arichardson, rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D46822

llvm-svn: 338385