Add a link to the Elf_Brandinfo into the struc proc.

To allow the ABI to make a dicision based on the Brandinfo add a link
to the Elf_Brandinfo into the struct proc. Add a note that the high 8 bits
of Elf_Brandinfo flags is private to the ABI.

Note to MFC: it breaks KBI.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D30918
MFC after: 2 weeks