[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