[RISCV] Add FreeBSD targets

Reviewers: asb

Reviewed By: asb

Subscribers: simoncook, s.egerton, lenary, psnobl, benna, mhorne, emaste, kito-cheng, shiva0217, rogfer01, rkruppe, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57795

Patch by James Clarke (jrtc27)

llvm-svn: 367557