libstdc++: Make std::chrono::current_zone() default to UTC

This is consistent with the behaviour of glibc, which assumes UTC when
/etc/localtime and TZ do not identify a valid time zone. The fallback
tzdb used when no valid tzdata exists always contains the UTC zone, so
this change means we have a valid tzdb and valid current zone even in
the degenerate case.

With this default we no longer need the AIX-specific kluge to try and
identify TZ values specifying a 0-offset zone. We can just use the UTC
default for those, as it has the same effect.

It's still possible for chrono::current_zone() to fail, because the user
could have provided a custom tzdata.zi file which doesn't contain the
UTC time zone, so the "UTC" default would fail to find a valid zone, and
throw an exception. That's just user error, they should not provide bad
data and expect reasonable behaviour.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (chrono::tzdb::current_zone()) Use "UTC" if
	current time zone cannot be determined.
	* testsuite/std/time/tzdb/1.cc: Remove conditions based on
	HAVE_TZDB macro and test all members unconditionally.