`fetch_xulrunner`, `dir_build`, and `build.sh` now take an optional `-a`
argument on Windows and Linux to specify the architecture to
fetch/build. If `-a` is omitted for `dir_build` (as in `build_and_run`),
it defaults to the current architecture.
xulrunner hashes are now computed for each architecture. This changes
some filenames in xulrunner/, so you may want to clean out some old
folders/files.
This removes the custom handling for CI, since it will now default to
the architecture of the test runner.
