To allow the ABI to make a dicision based on the Brandinfo add a link
to the Elf_Brandinfo into the struct sysentvec. Add a note that the high 8 bits
of Elf_Brandinfo flags is private to the ABI.
This can be done by adding a private_flag filed to the struct sysentvec,
but it requires a lot of more work - duplicate of sysentvec, add some fileds
to the struct sysentvec which is private to the ABI and so on.