Partially undo r351659, which unconditionally removed gets(3) from libc++.
Instead, pull in r371324 from upstream libc++ trunk (by me):
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
This makes these changes more MFCable.