Mono runtime fails to build cross-compilers for x86/x64 on arm/arm64

[directhex]

Splitting out the issues described in #74175, especially the comments #74175 (comment) and #74175 (comment)

As-is, we cannot build cross-compilers from arm64 to x64, so we cannot build cross-compilers for osx-arm64 to iOS, tvOS, Android, or Mac Catalyst. The same problem applies for Linux cross-compilers to Android. The build system is ready on the mono.proj etc side, it's problems with the Mono runtime itself.

[rdavisau]

In case it's useful, unscientific testing on my M1 showed using arm64 binaries for mono-aot-cross and friends made the aot compile task 30%-35% faster across a variety of projects.

[lambdageek]

After #74715 and #89027 I think this is our status now (updating the table from #74175 (comment)):

Build machine	Cross-compiler target	Executing machine	Status
linux-x64	browser-wasm	linux-arm64	☑
linux-x64	android-x64	linux-arm64	☑
linux-x64	android-arm64	linux-arm64	⚠ Builds, disabled in #74175
linux-x64	android-x86	linux-arm64	☑, ⚠ may need #89039 for local builds
linux-x64	android-arm	linux-arm64	⚠ Builds, disabled in #74175
win-x64	browser-wasm	win-arm64	☠ Does not build, removed from scope
win-x64	android-x64	win-arm64	☠ Does not build, removed from scope
win-x64	android-arm64	win-arm64	☠ Does not build, removed from scope
win-x64	android-x86	win-arm64	☠ Does not build, removed from scope
win-x64	android-arm	win-arm64	☠ Does not build, removed from scope
osx-x64	browser-wasm	osx-arm64	☑
osx-x64	android-x64	osx-arm64	☑
osx-x64	android-arm64	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	android-x86	osx-arm64	☑
osx-x64	android-arm	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	ios-arm64	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	iossimulator-arm64	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	iossimulator-x64	osx-arm64	☑
osx-x64	iossimulator-x86	osx-arm64	❌ Does not build - ❓ is this still supported
osx-x64	ios-arm	osx-arm64	⚠ Builds, disabled in PR - ❓ is this still supported
osx-x64	tvos-arm64	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	tvossimulator-arm64	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	tvossimulator-x64	osx-arm64	☑
osx-x64	maccatalyst-arm64	osx-arm64	⚠ Builds, disabled in #74175
osx-x64	maccatalyst-x64	osx-arm64	☑

I'm not sure what "disabled in #74175 means" - I think it means the CI pipelines were disabled. Some of them may be back on as part of #89027 @directhex

[directhex]

Just meant "disabled in the PR I'm trying to get merged"

Do we still support any scenarios where iossimulator-x86 or ios-arm matter? IOS dropped support for 32-bit ARM with iOS 11 (2017). I can't easily determine when x86 support for the simulator was dropped, but I imagine it's the same thing if the whole-ass OS lost 32-bit support at once.

What's the minimum iOS version we support?