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This patch adds common changes to support AArch32 state in
BL1 and BL2. Following are the changes:

* Added functions for disabling MMU from Secure state.
* Added AArch32 specific SMC function.
* Added semihosting support.
* Added reporting of unhandled exceptions.
* Added uniprocessor stack support.
* Added `el3_entrypoint_common` macro that can be
  shared by BL1 and BL32 (SP_MIN) BL stages. The
  `el3_entrypoint_common` is similar to the AArch64
  counterpart with the main difference in the assembly
  instructions and the registers that are relevant to
  AArch32 execution state.
* Enabled `LOAD_IMAGE_V2` flag in Makefile for
  `ARCH=aarch32` and added check to make sure that
  platform has not overridden to disable it.

Change-Id: I33c6d8dfefb2e5d142fdfd06a0f4a7332962e1a3

This patch adds the essential AArch32 architecture helpers
arch.h and arch_helpers.h and modifies `_types.h` to add AArch32
support.

A new build option `ARCH` is defined in the top level makefile to
enable the component makefiles to choose the right files based on the
Architecture it is being build for. Depending on this flag, either
`AARCH32` or `AARCH64` flag is defined by the Makefile. The default
value of `ARCH` flag is `aarch64`. The AArch32 build support will be
added in a later patch.

Change-Id: I405e5fac02db828a55cd25989b572b64cb005241