ShellPkg: Fix ARM build errors.

submitted-by: Olivier Martin olivier.martin@arm.com>
reviewed-by: Jaben Carsey <Jaben.Carsey@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14400 6f19259b-4bc3-4df7-8a09-765794883524