After fixing some issues with inline assembly in the C backend, the std
cleanups have the side effect of making these functions compatible with
the backend, allowing it to be used on linux without linking libc.
- Fix unwindFrame using the previous FDE row instead of the current one
- Handle unwinding through noreturn functions
- Add x86-linux getcontext
- Fixup x86_64-linux getcontext not restoring the fp env
- Fix start_addr filtering on x86-windows
There are still a few occurrences of "stage1" in the standard library
and self-hosted compiler source, however, these instances need a bit
more careful inspection to ensure no breakage.