Make devd shared now that libc++ is in /lib

Commit 5e6a2d6eb220 moved libc++ from /usr/lib to /lib, so we no longer
have an interval during boot when it is not available (before /usr is
mounted). We no longer need to force devd to be statically linked.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37409