NDK r18 drops GCC toolchains and some STL types. We need to choose a
clang toolchain by default when no gcc toolchains are available. Switch
the STL type default to `c++_static` when the old `gnustl_static`
default is not available.
Update the test suite to not run tests for STL types that do not exist.
Also do not expect the gcc toolchain `cpp` tool to be available because
r18 does not provide it. Also teach it to tolerate `gcc -dumpmachine`
output like `arm--linux-android` that differs from the toolchain prefix.
Fixes: #18301
The `armeabi` ABI is no longer available, so we can no longer use it by
default unconditionally. Instead detect all available ABIs and choose
the oldest arm ABI that is available.
Also update the test suite to account for the lack of `armeabi` support
and pass as of Android NDK r17-beta2.
Add missing "unwind" and "atomic" libraries needed for this combination.
See `${ndk}/sources/cxx-stl/llvm-libc++/libs/armeabi/libc++.a` for the
libraries the NDK uses.
Issue: #16380
