m77: explicily use android 21 api level linkers for now
[target.aarch64-linux-android]
-linker = "aarch64-linux-android-clang"
+linker = "aarch64-linux-android21-clang"
[target.x86_64-linux-android]
-linker = "x86_64-linux-android-clang"
+linker = "x86_64-linux-android21-clang"
[target.i686-linux-android]
-linker = "i686-linux-android-clang"
+linker = "i686-linux-android21-clang"
