[libunwind][RISCV] Add 64-bit RISC-V support

Summary:
Add unwinding support for 64-bit RISC-V.

This is from the FreeBSD implementation with the following minor
changes:

- Renamed and renumbered DWARF registers to match the RISC-V ABI [1]
- Use the ABI mneumonics in getRegisterName() instead of the exact
   register names
- Include checks for __riscv_xlen == 64 to facilitate adding the 32-bit
   ABI in the future.

[1] https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

Patch by Mitchell Horne (mhorne)

Reviewers: lenary, luismarques, compnerd, phosek

Reviewed By: lenary, luismarques

Subscribers: arichardson, sameer.abuasal, abidh, asb, aprantl, krytarowski, simoncook, kito-cheng, christof, shiva0217, rogfer01, rkruppe, PkmX, psnobl, benna, lenary, s.egerton, luismarques, emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D68362

0 files changed, 0 insertions, 0 deletions