Add CST (China Standard Time) to our lists of timezone abbreviations.

For some reason this seems to have been missed when the lists in
src/timezone/tznames/ were first constructed.  We can't put it in Default
because of the conflict with US CST, but we should certainly list it among
the alternative entries in Asia.txt.  (I checked for other oversights, but
all the other abbreviations that are in current use according to the IANA
files seem to be accounted for.)  Noted while responding to bug #12326.