build-hafnium.sh: use clang-llvm toolchain

Hafnium is built using clang-llvm toolchain, since this toolchain
install path is defined in the common configuration file for
build-scripts, this change makes use of such definitions.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: I3d8df9433de5226c3e55778e88ed46056ac85554