Make structure padding explicit in EFI_MEMORY_DESCRIPTOR

The EFI memory descriptor 64-bit aligns PhysicalStart on both 32- and
64-bit platforms. Make the padding explicit for i386 EFI.

Submitted by: Siva Mahadevan <smahadevan@freebsdfoundation.org>
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11301