arm64: big-endian: add big-endian support to top-level arch Makefile

arm64: big-endian: add big-endian support to top-level arch Makefile

This patch adds big-endian support to the AArch64 top-level Makefile.
This currently just passes the relevant flags to the toolchain and is
predicated on a Kconfig option that will be introduced later on.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit a0974e6e217aead196033d72f898e2acb575304d)

Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>