Reapply: move libc++ from /usr/lib to /lib

As with other runtime components like libc or libcxxrt.

If desired we can stop linking devd statically after this change (to
achive approximately no net change in required root filesystem size).

We must set SHLIBDIR with ?= before including <src.opts.mk>, otherwise
that will have set SHBLIDIR to its default value of /usr/lib.

Otherwise, "make delete-old-libs" would suggest to delete libc++.so.1
from /usr/lib, while there was not yet a copy in /lib.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33123