Extends support for arm64e to macOS & visionOS platforms

[pmattos]

Add arm64e support for macOS, iOS, and visionOS apps.

Motivation:

SwiftPM currently only supports building arm64e packages for iOS.

In Xcode 26 we added support to building packages for arm64e targeting macOS & visionOS platforms too.

Modifications:

Conditionally adds "arm64e" to the ARCHS build settings in the PackagePIFBuilder used to interface with Swift Build.

Result:

PIF builder nows had the capability of builds packages for arm64e. Note that this is currently disabled in SwiftPM, for now.

I validated this by building a simple macOS executable for arm64e:

$ swift package init --type executable --name HelloWorld
$ swift run swift-build --package-path HelloWorld --build-system swiftbuild

...and then I inspected the generated binary:

$ lipo -archs HelloWorld/.build/arm64-apple-macosx/Products/Debug/HelloWorld 
arm64e