[libc++] Re-enable std::pair trivial copy constructor for FreeBSD >= 14

After many years of using the really old std::pair ABI which did not yet
have a trivial copy constructor, FreeBSD 14 and later will finally get
rid of it. Only use the old ABI for FreeBSD 13 and earlier.

Note: on the FreeBSD side, we will bump our libc++.so version for this,
and keep an old compatibility library in a separate package.

Differential Revision: https://reviews.llvm.org/D126462

Author: DimitryAndric

libcxx/include/__config

@@ -164,7 +164,7 @@
 #    endif
 // Feature macros for disabling pre ABI v1 features. All of these options
 // are deprecated.
-#    if defined(__FreeBSD__)
+#    if defined(__FreeBSD__) && __FreeBSD__ < 14
 #      define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #    endif
 // For XCOFF linkers, we have problems if we see a weak hidden version of a symbol