[Sanitizers] Add support for RISC-V 64-bit

Summary:
This has been tested with gcc trunk on openSUSE Tumbleweed on the HiFive Unleashed.

Patch by Andreas Schwab (schwab)

Reviewers: luismarques

Reviewed By: luismarques

Subscribers: mhorne, emaste, luismarques, asb, mgorny, fedor.sergeev, simoncook, kito-cheng, shiva0217, rogfer01, rkruppe, lenary, s.egerton, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D66870