loader: want mechanism to avoid RA with bcache

While we have mechanisms in place to protect ourselves against the read
behind the disk end, there is still one corner case. As the GPT
partition table has backup table at the end of the disk, and we yet
do not know the size of the disk (if the wrong size is provided by the
firmware/bios), we need to limit the reads to avoid read ahead in such case.

Note: this update does add constant into stand.h, so the incremental build
will need to get local stand.h updated first.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D10187