refactor(build): simplify ENABLE_LTO checking

We do not support LTO with AArch32. The standard way to enforce this is
by having a rule to fail the build when this happens. There are no
AArch32 platforms which (incorrectly) try to enable LTO, so add this
rule. The benefit is that we no longer have to check the ENABLE_LTO with
ARCH whenever it is used.

Change-Id: Ic4086a1f5122269bda1d75bd4474b98fde35b5af
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
2 files changed