[libc++][Android] Support libc++ testing on Android (#69274)

I could probably break this commit into more pieces.

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This patch adds libc++ support for Android L (Android 5.0+) and up,
tested using the Android team's current compiler, a recent version of
the AOSP sysroot, and the x86[-64] Android Emulator.

CMake and Lit Configuration:

Add runtimes/cmake/android/Arch-${ARCH}.cmake files that configure CMake
to cross-compile to Android without using CMake's built-in NDK support
(which only works with an actual packaged NDK).

Add libcxx/cmake/caches/AndroidNDK.cmake that builds and tests libc++
(and libc++abi) for Android. This file configures libc++ to match what
the NDK distributes, e.g.:
- libc++_shared.so (includes libc++abi objects, there is no
libc++abi.so). libunwind is linked statically but not exported.
 - libc++_static.a (does not include libc++abi) and libc++abi.a
 - `std::__ndk1` namespace
- All the libraries are built with `__ANDROID_API__=21`, even when they
are link...