Teach procstat -x about the AT_EHDRFLAGS ELF auxiliary vector.

Submitted by: Shawn Webb <lattera@gmail.com>
Reviewed by: brooks
Obtained from: HardenedBSD (bb71e9889d1362df01c2e5162e84cd7a4fc029c8)
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D17067