[bootstrap] Prefix -F with -Xcc for corelibs

Author: aciidgh

Utilities/bootstrap

@@ -254,7 +254,7 @@ class Target(object):
 
         if args.foundation_path:
             import_paths.append(args.foundation_path)
-            other_args.extend(["-F", os.path.join(args.foundation_path, "CoreFoundation-prefix", "System", "Library", "Frameworks")])
+            other_args.extend(["-Xcc", "-F" + os.path.join(args.foundation_path, "CoreFoundation-prefix", "System", "Library", "Frameworks")])
             import_paths.append(os.path.join(args.foundation_path, "swift"))
         if args.libdispatch_build_dir:
             import_paths.append(os.path.join(args.libdispatch_build_dir, "src"))