Android: Add support for NDK r18

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
17 jobs for android-ndk-r18