[builtins] Build for arm64e for Darwin

https://github.com/apple/swift/pull/30112/ makes the Swift standard
library for iOS build for arm64e. If you're building Swift against your
own LLVM, this in turn requires having the builtins built for arm64e,
otherwise you won't be able to use the builtins (which will in turn lead
to an undefined symbol for `__isOSVersionAtLeast`). Make the builtins
build for arm64e to fix this.

Differential Revision: https://reviews.llvm.org/D76041