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

Modules/Platform/Android-Common.cmake

@@ -32,8 +32,10 @@ if(CMAKE_ANDROID_NDK)
         )
     endif()
     unset(_ANDROID_STL_TYPE_FOUND)
-  else()
+  elseif(IS_DIRECTORY ${CMAKE_ANDROID_NDK}/sources/cxx-stl/gnu-libstdc++)
     set(CMAKE_ANDROID_STL_TYPE "gnustl_static")
+  else()
+    set(CMAKE_ANDROID_STL_TYPE "c++_static")
   endif()
 
   unset(_ANDROID_STL_TYPES)