[TIMOB-18064] Enhances build script to allow more flexibility

This PR enhances the build script to allow more flexibility in building JavaScriptCore in different configurations, and helps my efforts as I work on [TIMOB-18064](https://jira.appcelerator.org/browse/TIMOB-18064).

To test this PR, run the `build.sh` script and go and grab a cup of coffee, because it takes at least 12 minutes to build all of the configurations. If you just want to see what would execute without actually building anything, then change line 26 from this
```bash
echo "${cmd}" && eval "${cmd}"
```
to this
```bash
echo "${cmd}" #&& eval "${cmd}"
```