Optimize Closure jar compilation

As noted in the [Closure FAQ](https://code.google.com/p/closure-compiler/wiki/FAQ#What_are_the_recommended_Java_VM_command-line_options?) (and discussed [here](https://plus.google.com/104744871076396904202/posts/CEwkCSQzLfr) and [here](https://groups.google.com/forum/#!topic/closure-compiler-discuss/Xsdx7m0gQnQ)), there are some recommended flags to java that speed up Closure compilation, often quite significantly.

I've only tested the 64-bit flags (and only on Linux) as I don't currently have a 32-bit system set up.  The other reservation I would have is that the test for 32- vs 64-bit tests PHP and assumes the java binary will be the same.