Consistent UI for debugging internal compiler issues.

When emcc runs internal tools such as the JS compiler and the closure compiler we currently have different ways to enable the next level of debugging. We should probably decide on one of:

1. Recommend re-running with `-sVERBOSE` on the command line
2. Recommend re-running with `EMCC_DEBUG=1` in environment
3. Recommend re-running with `-save-temps`
4. Always save temps somewhere in case of error (avoid re-running at all?)

Currently (1) is used for JS compiler and (2) is used for closure compiler, but (3) is the standard gcc/clang name.