Refine check for `_LIBCPP_C_HAS_NO_GETS` on FreeBSD
Refine check for `_LIBCPP_C_HAS_NO_GETS` on FreeBSD

Summary:
In D67316 we added `_LIBCPP_C_HAS_NO_GETS` to signal that the C library
does not provide `gets()`, and added a test for FreeBSD 13 or higher,
using the compiler-defined `__FreeBSD__` macro.

Unfortunately this did not work that well for FreeBSD's own CI process,
since the gcc compilers used for some architectures define `__FreeBSD__`
to match the build host, not the target.

Instead, we should use the `__FreeBSD_version` macro from the userland
header `<osreldate.h>`, which is more fine-grained.  See also
<https://reviews.freebsd.org/D22034>.

Reviewers: EricWF, mclow.lists, emaste, ldionne

Reviewed By: emaste, ldionne

Subscribers: dexonsmith, bsdjhb, krytarowski, christof, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D69174

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@375340 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx