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>