stand: Introduce md_align

When we're loading metadata, we need to align it in a certain way. Right
now that way is hard-coded to be PAGE_SIZE. Rather than do the actual
physical thing in all these places, move this into a wrapper routine. We
may want to load a 16k kernel and align all these on 16k or a 4k kernel
and align on 4k on aarch64 (today you have to compile the loader with
the right page size). This will also reduce the number of places we
might have to touch to do that.

Sponsored by: Netflix
Reviewed by: tsoome, jhibbits
Differential Revision: https://reviews.freebsd.org/D50585