build(make): use clang binutils to compile

Utilise LLVM binutils during compilation of TF-A instead of the default
utilities provided by the host. The Makefile looks in the path provided
for the current toolchain and only checks for them on the host if none
exist in this path. If the utilities don't exist in either place then
the build fails.

Change-Id: I6af2aa09ea3c8743cf3df6600d9760d909d76647
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>