compiler: include error trace in all functions, implement for x86_64 backend

[mlugg]

This PR does two things:

• Simplify the error return trace to be passed between all functions, not just those that return errors.
  • This makes it possible to (for instance) call std.debug,panic and have it receive the error return trace even though it doesn't return an error.
  • This is also probably a more efficient lowering in practice, since it avoids having many stack frames which have to set up the trace. Broadly speaking, this now only happens at "entry points".
  • This means std.builtin.returnError also gets an implicit error trace parameter, so it no longer needs to take it in explicitly, saving one AIR instruction per error return.
• Implement the error return trace on the x86_64 backend (by @jacobly0), hence enabling the error_return_trace backend feature.
  • We aim to keep the pointer in r9 at all times
  • This makes the lowering more efficient than what the LLVM backend tends to do; for instance, function calls typically have no overhead for passing the trace, since the caller has it in r9 and the callee expects it in r9

Contributes to #21530.