Because there's no such thing as API 19 ARM64. 64-bit support was added in android-21.
CMake supports this because our toolchain file was modeled off of a popular option that was commonly used at the time and that's what it did. ndk-build does it because you build multiple ABIs in a single invocation. In both cases, the build automatically pulls the API level up to 21 for 64-bit targets.
Standalone toolchains are for exactly one architecture, so they give an error if you specify an API level that is not supported by that architecture since it was likely a mistake.
