ELF auxargs: reserve a slot for CheriABI use

Reserve slot 37 to avoid flag days downstream.

We currently use this slot in CheriBSD to maintain counters in userspace
while providing access to them vis sysctl. CHERI support isn't landing
in FreeBSD quite yet (probably for 16 unless hardware access accelerates
radically), but it seems highly likely at this point so reserving slots
will limit future diffs in both directions.

Reviewed by: jhb
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D46060