Android: Fix binutils selection with NDK r19+ unified toolchain

In commit 97bca2f9 (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
7 jobs for android-ndk-r19-binutils
Status Job ID Name Coverage
  External
passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

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