xtf: Perform misc code cleanup

xtf: Perform misc code cleanup

 * Define macro ALIGN to set alignment.  Header file asm_macros.h should not
   contain architecture specific code.  Replace hardcoded alignment value with
   a call to macro ALIGN.

 * Move declaration of shared_info into xtf/traps.h.  Declaration of
   shared_info structure should be placed in xtf/traps.h as it is a common
   declaration for all the possible architectures.

 * Do not protect including <arch/barrier.h>.  We should always include
   <arch/barrier.h> and rely on the compiler to throw an error. This would
   avoid increasing #if protection each time we would add a new architecture.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>