Android: Use unified toolchain in NDK r19+

Brad Kingrequested to merge
brad.king/cmake:android-ndk-r19 into master

The NDK build system now uses only a single toolchain in

<ndk>/toolchains/llvm/prebuilt/<host>

Its compilers are always bin/{clang,clang++} and its binutils are always bin/<triple>-*. It is a standalone toolchain:

  • The Anrdoid API level is specified at the end of --target=.
  • The standard library may be specified via -stdlib=.
  • No need to pass system includes or libraries explicitly.
  • No need to pass --sysroot or -gcc-toolchain.

Teach CMake to recognize NDK versions that have a unified toolchain with its own sysroot and use the above approach.

Fixes: #18739 (closed)

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