Android: Add support for x86_64 architecture

Like arm64v8, this is only supported by API 21 and later,
so we enforce 21 as min API for x86_64.

Part of #25030.

platform/android/SCsub

@@ -152,6 +152,8 @@ elif env['android_arch'] == 'arm64v8':
     lib_arch_dir = 'arm64-v8a'
 elif env['android_arch'] == 'x86':
     lib_arch_dir = 'x86'
+elif env['android_arch'] == 'x86_64':
+    lib_arch_dir = 'x86_64'
 else:
     print('WARN: Architecture not suitable for embedding into APK; keeping .so at \\bin')