MdeModulePkg/RegularExpressionDxe: Fix GCC build error

Fix GCC build error on AARCH64 system.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Nick Ramirez <nramirez@nvidia.com>
Signed-off-by: Nickle Wang <nicklew@nvidia.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>