Remove ::gets for FreeBSD 13 and later

Summary:
In https://svnweb.freebsd.org/changeset/base/351659 @emaste removed gets() from
FreeBSD 13's libc, and our copies of libc++ and libstdc++.  In that change, the
declarations were simply deleted, but I would like to propose this conditional
test instead.

Reviewers: EricWF, mclow.lists, emaste

Reviewed By: mclow.lists

Subscribers: krytarowski, christof, ldionne, emaste, libcxx-commits

Differential Revision: https://reviews.llvm.org/D67316

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@371324 91177308-0d34-0410-b5e6-96231b3b80d8