Add unified headers support to the libc++ tests.

Test: ./test_libcxx.py --unified-headers -a armeabi-v7a -p 9
Bug: android/ndk#120
Change-Id: Ib5e6ad3ba647aea3086dd4b91046efcc988c196a

test_libcxx.py

@@ -32,6 +32,9 @@ def parse_args():
     parser.add_argument(
         '-p', '--platform', required=True, type=int,
         help='API level to build against.')
+    parser.add_argument(
+        '--unified-headers', action='store_true', default=False,
+        help='Use NDK unified headers.')
     parser.add_argument(
         '-t', '--timeout', default=300, type=int,
         help='Per-test timeout in seconds.')
@@ -101,6 +104,7 @@ def main():
 
     lit_args = [
         lit_path, '-sv', '--param=device_dir=' + device_dir,
+        '--param=unified_headers={}'.format(args.unified_headers),
         '--timeout={}'.format(args.timeout)
     ] + extra_args
     if not have_filter_args: