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.