libllvm does not create a native llvm-config

Cross compiling llvm builds a native (host=build) version of llvm-config as a make step. But in termux, this is not the case. The native llvm-config is built for target host. The native build takes place in $TERMUX_PKG_BUILDDIR/NATIVE. I was unable to find the root cause, but, the native build also uses $TERMUX_STANDALONE_TOOLCHAIN/bin/clang++. I suspect that this may be a bug in the cross compilation mechanism of cmake itself.

This is required to build rustc and other packages dependent on llvm-config.