ship stage2 instead of stage1

[andrewrk]

Here's a tracking issue for when we can completely replace stage1 with self-hosted in official builds of Zig.

• add debug info to the LLVM backend (open PR: stage2 LLVM debug info #11085)
• implement runtime safety checks
• get compilation errors into an acceptable state - this will involve reworking the compile error test harness
• adjust the default backend to be LLVM until other backends reach full behavior test coverage
• suspend/resume/async/await implemented in stage2
• all behavior tests passing with the LLVM backend (current status: 82%)
• all std lib tests passing with the LLVM backend
• full test suite passing with the LLVM backend
• check all the TODO comments in stage2 source code and convert relevant ones into issues blocking this one

[andrewrk]

duplicate of #89