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
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
arm--linux-android that differs from the toolchain prefix.
Fixes: #18301 (closed)