Add arm64_32 support for Swift

Commit the platform definition and build script work necessary to
cross-compile for arm64_32.

arm64_32 is a variant of AARCH64 that supports an ILP32 architecture.

test/Inputs/SmallStringTestUtilities.swift

@@ -71,7 +71,7 @@ extension _SmallString {
  
 #if _runtime(_ObjC)
   init?(_cocoaString ns: NSString) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(arm64_32)
     return nil
 #else
     guard _isObjCTaggedPointer(ns) else { return nil }