ArmPkg/ArmExceptionLib: use static buffer for sp_el0
    
    The exception library is also used in DxeMain before memory services
    are available, and AllocatePages() will fail in this case and cause
    sp_el0 remains 0. Then if any exception occurs before CpuDxe driver is
    loaded, a recursive exception will be trigged by page translation
    fault for sp = 0 - 0x130.
    
    Use static buffer instead to fix this issue.
    
    Signed-off-by: Heyi Guo <guoheyi@linux.alibaba.com>
    Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>