[RISCV GlobalISel] Adding initial GlobalISel infrastructure

[RISCV GlobalISel] Adding initial GlobalISel infrastructure

Summary:
Add an initial GlobalISel skeleton for RISCV. It can only run ir translator for `ret void`.

Patch by Andrew Wei

Reviewers: asb, sabuasal, apazos, lenary, simoncook, lewis-revill, edward-jones, rogfer01, xiangzhai, rovka, Petar.Avramovic, mgorny, dsanders

Reviewed By: dsanders

Subscribers: pzheng, s.egerton, dsanders, hiraditya, rbar, johnrusso, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, psnobl, benna, Jim, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65219

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369467 91177308-0d34-0410-b5e6-96231b3b80d8