Introduce src.conf knob to build userland with retpoline

WITH_RETPOLINE enables -mretpoline vulnerability mitigation in userland
for CVE-2017-5715.

Reported by: Peter Malcom
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17421