Switch AARCH32/AARCH64 to __aarch64__

NOTE: AARCH32/AARCH64 macros are now deprecated in favor of __aarch64__.

All common C compilers pre-define the same macros to signal which
architecture the code is being compiled for: __arm__ for AArch32 (or
earlier versions) and __aarch64__ for AArch64. There's no need for TF-A
to define its own custom macros for this. In order to unify code with
the export headers (which use __aarch64__ to avoid another dependency),
let's deprecate the AARCH32 and AARCH64 macros and switch the code base
over to the pre-defined standard macro. (Since it is somewhat
unintuitive that __arm__ only means AArch32, let's standardize on only
using __aarch64__.)

NOTE: This change is based on below TFA commit
https://github.com/ARM-software/arm-trusted-firmware/commit/402b3cf8766fe2cb4ae462f7ee7761d08a1ba56c



Signed-off-by: Deepika Bhavnani <deepika.bhavnani@arm.com>
Change-Id: If2c3dbaeb01d4a9d8cfd95d906e5eaf4ae94417f