This patch enables LTO for TF-A when compiled with GCC.
LTO is disabled by default and is enabled by
ENABLE_LTO=1 build option.

LTO is enabled only for aarch64 as there seem to be
a bug in the aarch32 compiler when LTO is enabled.

The changes in the makefiles include:
- Adding -flto and associated flags to enable LTO.
- Using gcc as a wrapper at link time instead of ld.
  This is recommended when using LTO as gcc internally
  takes care of invoking the necessary plugins for LTO.
- Adding switches to pass options to ld.
- Adding a flag to disable fix for erratum cortex-a53-843419
  unless explicitly enabled. This is needed because GCC
  seem to automatically add the erratum fix when used
  as a wrapper for LD.

Additionally, this patch updates the TF-A user guide with
the new build option.

Signed-off-by: zelalem-aweke <zelalem.aweke@arm.com>
Change-Id: I1188c11974da98434b7dc9344e058cd1eacf5468