Only RV64GC is supported.
Bug: fuchsia:127655
Tested: `python build/run_tests.py` on RISC-V emulator
Tested: Created minidump via self-induced crash on RISC-V emulator,
ran through Breakpad stackwalker
Change-Id: I713797cd623b0a758269048e01696cbce502ca6c
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/4581050
Reviewed-by: Joshua Peraza <jperaza@chromium.org>