[macos] swift binary running under rosetta

[iloveitaly]

Not sure if there is a intentional reason behind this, but I could fix this up.

[ErikBjare]

Probably only for backwards-compatibility reasons, like the rest of the build (I assume).

[iloveitaly]

I think we could do a multi-arch build for the swift binary. Will try to get that done.

[TimeToBuildBob]

The build currently uses a plain swiftc $^ -o $@ without any architecture flags (Makefile line), so it compiles for whatever the build host's native architecture is. On Intel CI runners, this produces x86_64 binaries that run under Rosetta on Apple Silicon.

Fix is straightforward — use a universal binary:

aw_watcher_window/aw-watcher-window-macos: aw_watcher_window/macos.swift
	swiftc $^ -o $@ -target arm64-apple-macosx11.0 -target x86_64-apple-macosx10.15

Or if the CI runner supports it, build a fat binary:

swiftc macos.swift -o aw-watcher-window-arm64 -target arm64-apple-macosx11.0
swiftc macos.swift -o aw-watcher-window-x86_64 -target x86_64-apple-macosx10.15
lipo -create -output aw-watcher-window-macos aw-watcher-window-arm64 aw-watcher-window-x86_64

The GitHub Actions workflow for aw-qt already builds on macos-14 (which is arm64), so the swift binary should come out as native arm64 now. This may already be resolved in recent builds — @iloveitaly did you have a chance to look at this?