Android: Use unified toolchain in NDK r19+

The NDK build system now uses only a single toolchain in

    <ndk>/toolchains/llvm/prebuilt/<host>

Its compilers are always `bin/{clang,clang++}` and its binutils are
always `bin/<triple>-*`.  It is a standalone toolchain:

* The Anrdoid API level is specified at the end of `--target=`.
* The standard library may be specified via `-stdlib=`.
* No need to pass system includes or libraries explicitly.
* No need to pass `--sysroot` or `-gcc-toolchain`.

Teach CMake to recognize NDK versions that have a unified
toolchain with its own sysroot and use the above approach.

Fixes: #18739

Help/manual/cmake-toolchains.7.rst

@@ -399,8 +399,10 @@ Configure use of an Android NDK with the following variables:
   be false unless using a NDK that does not provide unified headers.
 
 :variable:`CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION`
-  Set to the version of the NDK toolchain to be selected as the compiler.
-  If not specified, the default will be the latest available GCC toolchain.
+  On NDK r19 or above, this variable must be unset or set to ``clang``.
+  On NDK r18 or below, set this to the version of the NDK toolchain to
+  be selected as the compiler.  If not specified, the default will be
+  the latest available GCC toolchain.
 
 :variable:`CMAKE_ANDROID_STL_TYPE`
   Set to specify which C++ standard library to use.  If not specified,

Help/variable/CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION.rst

@@ -3,7 +3,11 @@ CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION
 
 When :ref:`Cross Compiling for Android with the NDK`, this variable
 may be set to specify the version of the toolchain to be used
-as the compiler.  The variable must be set to one of these forms:
+as the compiler.
+
+On NDK r19 or above, this variable must be unset or set to ``clang``.
+
+On NDK r18 or below, this variable must be set to one of these forms:
 
 * ``<major>.<minor>``: GCC of specified version
 * ``clang<major>.<minor>``: Clang of specified version