ArmPkg/AsmMacroIoLib: Fixed the global variables initialization

ArmPkg/AsmMacroIoLib: Fixed the global variables initialization

The top of the stack always points to 'stack_base + stack_size'.
But the stack pointer is decremented before writing to the stack.
It means the top byte of the stack is actually 'stack_base + stack_size - 1'.

The initialization should also decrement the stack pointer before
zero'ing the memory (pre-indexed addressing).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16518 6f19259b-4bc3-4df7-8a09-765794883524