Android: Fix binutils selection with NDK r19+ unified toolchain

In commit 97bca2f9fa (Android: Use unified toolchain in NDK r19+,
2019-07-26, v3.16.0-rc1~342^2) we hard-coded use of the unified
toolchain for NDK r19+ and skipped most of the old detection logic.
However, in that fast path we left out setting `_CMAKE_TOOLCHAIN_PREFIX`
for `CMakeFindBinutils` to select the matching binutils.  Add it.

Fixes: #20038

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

@@ -23,6 +23,7 @@ if(CMAKE_ANDROID_NDK_TOOLCHAIN_UNIFIED)
   set(_ANDROID_TOOL_CXX_COMPILER_EXTERNAL_TOOLCHAIN "")
   set(_ANDROID_TOOL_CXX_TOOLCHAIN_PREFIX "${CMAKE_ANDROID_NDK_TOOLCHAIN_UNIFIED}/bin/${CMAKE_ANDROID_ARCH_TRIPLE}-")
   set(_ANDROID_TOOL_CXX_TOOLCHAIN_SUFFIX "${_ANDROID_HOST_EXT}")
+  set(_CMAKE_TOOLCHAIN_PREFIX "${CMAKE_ANDROID_ARCH_TRIPLE}-")
   return()
 endif()