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 (closed)