Mk/Uses: Add elfctl

Add USES=elfctl to change an ELF binary's feature control note.

Example:

USES= elfctl
ELF_FEATURES= +noaslr,wxneeded:foo

Reported by: emaste
Approved by: brooks, tcberner (portmgr)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34125