Support AIX's TZDB location

[ecnelises]

This is to support AIX operating system. Since it's not been official recognized target of rust, this PR is in draft status.

[djc]

I'm happy to merge this independent of whether it's an official target given the minimal changes (and it seems unlikely the OS identifier would change).

[djc]

(BTW, you may also want to submit a PR for this to the 0.4.x branch.)

[esheppa]

Thanks for this @ecnelises. By any chance do you know how the current timezone is specified on AIX? It looks like one option is the TZ environment variable, but there may be others, however I've not been able to find any documentation. This would be useful as we could PR this to the iana-time-zone library which we depend on

[Kijewski]

@esheppa, OP is already working on a PR for iana-time-zone: strawlab/iana-time-zone#57. I linked some man pages in my first comment.