Build fat packages when shouldPackagesBuildForARM64e is set (#8969)

### Motivation:

I extended arm64e support for macOS & visionOS apps in my previous
PR #8837. When enabled, that patch *only* produced the arm64e slice
but...

The App Store will now accept arm64e for macOS, iOS, and visionOS —
arm64e is optional and off by default. But when you turn on arm64e
support for packages (ie, using `shouldPackagesBuildForARM64e` PIF
builder delegate method) we expect *both* slices in the binary now.

### Modifications:

Changes the PIF builder to update the ARCHS settings without
overriding it completely, ensuring we get all required slices (ie, a so
called fat binary).

### Result:

PIF builder nows has the capability of building packages for arm64 and
arm64e. (Note, for now, that this is currently disabled in SwiftPM.)

I validated this by building a simple macOS executable for arm64e (and
force enabling it in SwifPM):

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

...and then I inspected the generated binary:

    $ lipo -archs HelloWorld/.build/arm64-apple-macosx/Products/Release/HelloWorld 
    x86_64 arm64 arm64e

Debug builds. With ONLY_ACTIVE_ARCH = "YES" in debug builds, even
though you specify multiple architectures in ARCHS, it will still only
build for the active architecture during development. This is set to
"NO" for release builds.

Author: pmattos

Sources/SwiftBuildSupport/PackagePIFBuilder.swift

@@ -590,7 +590,7 @@ public final class PackagePIFBuilder {
                 default:
                     .init(from: arm64ePlatform)
                 }
-                settings.platformSpecificSettings[pifPlatform]![.ARCHS] = ["arm64e"]
+                settings.platformSpecificSettings[pifPlatform]![.ARCHS, default: []].append(contentsOf: ["arm64e"])
             }
         }