Since NDK commit 90ec78ffd96b87cd75d82575587ead14d6494df1 (Remove Clang
toolchain path from setup.mk, 2017-05-31) the `setup.mk` files for Clang
no longer hold the path to the `toolchains/llvm` directory.  It has been
the same since NDK r11, so use that as the default.

Fixes: #17253