Android: Use unified toolchain in NDK r19+
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
--sysrootor-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)