[android] A few tweaks for native compilation and to get more tests working

Now that CMAKE_HOST_SYSTEM_NAME and CMAKE_SYSTEM_NAME are set by default to
Android in the Termux app, make the needed tweaks. Some tests were adapted
to work natively on Android too, adds sys/cdefs.h to the Bionic modulemap,
and includes the start of native Android platform support in the build-script.

utils/swift_build_sdk_interfaces.py

@@ -402,9 +402,18 @@ def main():
             xfails = json.load(xfails_file)
 
     make_dirs_if_needed(args.output_dir, args.dry_run)
-    shared_output_lock = multiprocessing.Lock()
-    pool = multiprocessing.Pool(args.jobs, set_up_child,
-                                (args, shared_output_lock))
+    if 'ANDROID_DATA' not in os.environ:
+        shared_output_lock = multiprocessing.Lock()
+        pool = multiprocessing.Pool(args.jobs, set_up_child,
+                                    (args, shared_output_lock))
+    else:
+        # Android doesn't support Python's multiprocessing as it doesn't have
+        # sem_open, so switch to a ThreadPool instead.
+        import threading
+        shared_output_lock = threading.Lock()
+        from multiprocessing.pool import ThreadPool
+        pool = ThreadPool(args.jobs, set_up_child,
+                          (args, shared_output_lock))
 
     interface_framework_dirs = (args.interface_framework_dirs or
                                 DEFAULT_FRAMEWORK_INTERFACE_SEARCH_PATHS)