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)

Modules/Platform/Android/Determine-Compiler-NDK.cmake

@@ -35,7 +35,18 @@ elseif(CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION)
   endif()
   set(_ANDROID_TOOL_PATTERNS "*-${CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION}")
 else()
-  set(_ANDROID_TOOL_PATTERNS "*-[0-9].[0-9]")
+  # If we can find any gcc toolchains then use one by default.
+  # Otherwise we look for clang toolchains (e.g. NDK r18+).
+  file(GLOB _ANDROID_CONFIG_MKS_FOR_GCC
+    "${CMAKE_ANDROID_NDK}/build/core/toolchains/*-[0-9].[0-9]/config.mk"
+    "${CMAKE_ANDROID_NDK}/toolchains/*-[0-9].[0-9]/config.mk"
+    )
+  if(_ANDROID_CONFIG_MKS_FOR_GCC)
+    set(_ANDROID_TOOL_PATTERNS "*-[0-9].[0-9]")
+  else()
+    set(_ANDROID_TOOL_PATTERNS "*-clang")
+  endif()
+  unset(_ANDROID_CONFIG_MKS_FOR_GCC)
 endif()
 set(_ANDROID_CONFIG_MK_PATTERNS)
 foreach(base "build/core/toolchains" "toolchains")