Exception handling support

Currently the code generation seems to optimize away everything but the no-throw code path. What's needed for generating code for the rest of the IR code paths? (e.g. landingpad, etc.).

By working around the current backend limitations regarding atomics and TLS, I have the D language runtime working. Yet, many parts of the standard library will misbehave due to throw exceptions not being handled. This ends up impacting functionality that would normally work even without exceptions. Given that this is the only major issue that seems to remain for proper D RISC-V support, adding this functionality would be most appreciated.

The issue names no files or tests; start by locating the backend code-generation paths for exception-related IR such as landingpad, along with the existing handling for atomics and TLS. Verify the behavior against throwing code in the D runtime and standard library, and consider the work complete when exception paths are generated and affected functionality works on RISC-V.

Written by the indexing model from the issue text.