On Mac, in the CMake USE_BUGSPLAT logic, we created both xcarchive.zip (which
is what BugSplat wants to see) and secondlife-symbols-darwin -64.tar.bz2 (which
we don't think is used for anything). The tarball was posted to codeticket --
but why? If the point is to manually re-upload to BugSplat in case of failure,
we'll do better saving xcarchive.zip to codeticket.

For SL-19243, posting xcarchive.zip directly supports the goal of breaking out
the upload to BugSplat as a separate step.

Anyway, since xcarchive.zip is a superset of the tarball, the tarball can be
recreated from the zip file, whereas the zip file can't be recreated from the
tarball without opening the .dmg installer and extracting the viewer executable.

If the xcarchive.zip file exists (that is, on Mac), post that to codeticket or
GitHub, as applicable, instead of the tarball. In fact, in the USE_BUGSPLAT
case, don't even bother creating the tarball since we're going to ignore it.

Make the new build.sh logic that insists on BUGSPLAT_USER and BUGSPLAT_PASS
conditional on BUGSPLAT_DB.