md driver compat32: fix structure padding for arm, powerpc

Because the 32-bit md_ioctl structure contains 64-bit members, arm
and powerpc add padding to a multiple of 8. i386 doesn't do this.
The md_ioctl32 definition was correct for amd64/i386 without padding,
but wrong for arm64 and powerpc64. Make packed conditional on
amd64, and test for the expected size on non-amd64. Note that
mdconfig is used in the ATF test suite. Note, I verified the
structure size for powerpc, but was unable to test.

MFC after: 1 week
Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D41339
Discussed with: jhibbits