docs(prerequisites): use LLVM utilities to build with clang

Makefile updated to use LLVM utilities instead of GNU utilities when
compiling with clang. `CROSS_COMPILE` is not required since this
dependency has been removed.

Change-Id: I19706b84b9310e07935516681b86596c04ef8ad6
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>