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: https://llvm.org/svn/llvm-project/libcxx/trunk@375340 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -242,6 +242,7 @@
 
 #ifdef __FreeBSD__
 #  include <sys/endian.h>
+#  include <osreldate.h>
 #  if _BYTE_ORDER == _LITTLE_ENDIAN
 #    define _LIBCPP_LITTLE_ENDIAN
 #  else  // _BYTE_ORDER == _LITTLE_ENDIAN
@@ -1178,7 +1179,8 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 
 // Some systems do not provide gets() in their C library, for security reasons.
 #ifndef _LIBCPP_C_HAS_NO_GETS
-#  if defined(_LIBCPP_MSVCRT) || (defined(__FreeBSD__) && __FreeBSD__ >= 13)
+#  if defined(_LIBCPP_MSVCRT) || \
+      (defined(__FreeBSD_version) && __FreeBSD_version >= 1300043)
 #    define _LIBCPP_C_HAS_NO_GETS
 #  endif
 #endif