Android: Update toolchain selection for NDK r19
The <ndk>/build/core/toolchains/*/setup.mk files no longer have any
TOOLCHAIN_ROOT or TOOLCHAIN_VERSION fields. The TOOLCHAIN_ROOT
is now hard-coded in <ndk>/build/core/default-build-commands.mk.
The NDK build system now uses only a single toolchain in
<ndk>/toolchains/llvm/prebuilt/<host>The compilers are always bin/{clang,clang++} and the binutils are
always bin/$(TOOLCHAIN_NAME)-*. It does not use -gcc-toolchain.
For now CMake can continue to use -gcc-toolchain. In future work we
may be able to switch to what the NDK buildsystem now does. NDK r19
still provides the GCC toolchains (without the compilers) suitable for
use with -gcc-toolchain. They are located in
<ndk>/toolchains/<toolchain>/prebuilt/<host>Find a GCC toolchain for the target platform without TOOLCHAIN_ROOT
by globbing for one of the binutils.
Fixes: #18739 (closed)