ae41d5b3f7
Cleans up the logic for building gen_snapshot for macOS hosts. Also adds
a bit more documentation to the snapshot-related build targets.
Previously:
* gen_snapshot targeting macOS (on both x64, and arm64) were generated
via the `create_macos_gen_snapshots` target.
* gen_snapshot arm/arm64 builds (on a macOS host) were generated via the
`create_arm_gen_snapshot` target. The reason why this covers only
arm/arm64 is simply that those are the only platforms we generate
iOS/Android AOT builds for. x64 iOS implies a simulator build, but we
only support debug builds for the iOS simulator, which aren't AOT
builds, and thus don't require gen_snapshot.
These two conditions overlap in the case of arm64 (but not x64) target
builds. This separates the two cases cleanly. A later patch will merge
the two cases to use a single tool.
Note that NEITHER of these two scenarios mentions builds for macOS hosts
targeting Fuchsia. That's because this target is never invoked during
Fuchsia target builds. See:
75d2baf5fe/BUILD.gn (L93-L95)
Issue: https://github.com/flutter/flutter/issues/103386
Issue: https://github.com/flutter/flutter/issues/101138