Define _LIBCPP_HAS_TIMESPEC_GET for FreeBSD when appropriate

Summary:
FreeBSD got `timespec_get` support somewhere in the 12.x timeframe, but
the C++ version check in its system headers was written incorrectly.
This has now been fixed for both FreeBSD 13 and 12.

Add checks for the corresponding `__FreeBSD_version` values, to define
`_LIBCPP_HAS_TIMESPEC_GET` when the function is supported.

Reviewers: emaste, EricWF, ldionne, mclow.lists

Reviewed By: ldionne

Subscribers: arichardson, krytarowski, christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D71522

Author: DimitryAndric

libcxx/include/__config

@@ -342,6 +342,10 @@
 #    define _LIBCPP_HAS_ALIGNED_ALLOC
 #    define _LIBCPP_HAS_QUICK_EXIT
 #    define _LIBCPP_HAS_C11_FEATURES
+#    if __FreeBSD_version >= 1300064 || \
+       (__FreeBSD_version >= 1201504 && __FreeBSD_version < 1300000)
+#      define _LIBCPP_HAS_TIMESPEC_GET
+#    endif
 #  elif defined(__BIONIC__)
 #    define _LIBCPP_HAS_C11_FEATURES
 #    if __ANDROID_API__ >= 21