Install library symlinks atomically.

As we do for shared library binaries, pass -S to install(1) when
installing symlinks. Doing so helps avoid transient failures when
libraries are being reinstalled, which seems to be the root cause of
spurious libgcc_s.so link failures during CI builds.

PR: 233769
Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26453