Add TZDB support on FreeBSD

[mordante]

At the moment it seems FreeBSD does not install the database's source files. Once that is fixed we can adjust libc++'s implementation and test.

One known issue that needs to be solved is the how to get the current time zone.
@jrtc27 reported in #105634

"GCC falls back on reading /etc/timezone (for Debian and derivatives) and /var/db/zoneinfo (for FreeBSD). Whether that's sensible I don't know, but it's better than doing nothing."