The LLVM linker replaces the GNU linker as default for the link on Clang
builds. It is possible to override the default linker by setting the LD build
flag.

The patch also updates the TF-A doc.

Change-Id: Ic08552b9994d4fa8f0d4863e67a2726c1dce2e35
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>