Set explicitly the desired Darwin architecture for all CMake builds

[edymtt]

This would be needed to compile on Apple Silicon, so that CMake does not
infer the wrong architecture when we want to build for Intel (see
https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5291)

Addresses rdar://99587478

[edymtt]

@swift-ci please test

[azarovalex]

@edymtt big thanks for trying to fix the toolchain build👍

I tried a similar fix while was trying to fix #1167 but for some reason, CMake didn't allow passing CMAKE_OSX_ARCHITECTURES flag to the project containing Swift sources, maybe something was wrong with my setup, I've just retested your solution and it works like a charm, I can close #1167.

Please note that this script has another problem I addressed here: #1168, this is not yet merged.

[artemcm]

@edymtt the CI here does not run the CMake build, so please verify it independently either locally or via cross-repo testing.

[edymtt]

Did the verification both locally and at swiftlang/swift#60901

Checked that the generated toolchain is still building swift-driver fat

% lipo -archs Library/Developer/Toolchains/swift-PR-60901-291.xctoolchain/usr/bin/swift-driver
x86_64 arm64
% lipo -archs Library/Developer/Toolchains/swift-PR-60901-291.xctoolchain/usr/bin/swift-help  
x86_64 arm64