remove `-fstage1` option

[andrewrk]

After this commit, the self-hosted compiler does not offer the option to use stage1 as a backend anymore.

Merge blockers:

• #6025
• #12756
• #12880
• #13077

[jedisct1]

#12880 is major issue preventing stage2 from being used when targeting WebAssembly.

[Luukdegram]

#12880 is major issue preventing stage2 from being used when targeting WebAssembly.

I was unaware of that issue, but I'll happily investigate and fix the issue before this is merged.

[billzez]

Please don't take away stage1 until we can create multi-file modules again:

https://github.com/ziglang/zig/issues/13077

I have multiple projects that are blocked by this (and async/await).

[IridescentRose]

Also would like to bring up #12756 where stage2 actually regresses on the abilities of stage1 in terms of comptime assembly. In that thread it was also mentioned that this affects aarch64 and vastly increases the amount of code

[andrewrk]

Thanks all - I added more merge blockers in the PR description.

[Luukdegram]

#12880 is major issue preventing stage2 from being used when targeting WebAssembly.

#13396 :)

[nektro]

https://github.com/ziglang/zig/issues/12404 would like this to be added as a blocker please

[andrewrk]

Superseded by #13560. We will be temporarily regressing async/await/suspend/resume in master branch.