Add missing definition of ELF_MACHINE_OK, now used by gcore(1).
Add two sysctls for newly added tunables.

MFC after:	2 weeks
Split the XHCI TRB allocations into smaller parts, so that we don't
end up allocating contiguous busdma buffers above PAGE_SIZE bytes.

MFC after:	1 week
Tested by:	Ruslan Bukin <br@bsdpad.com>