Android: Fix binutils selection with NDK r19+ unified toolchain
In !3612 (merged) 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 (closed)
Backport: release