Reserve WXNEEDED ELF feature control flag

This will be used to tag binaries that require W+X mappings, in advance
of the ability to prevent W^X in mmap/mprotect.

There is still some discussion about the flag's name, but the ABI won't
change even if the name does (as kib pointed out in the review).

Reviewed by: csjp, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23909