Add V8_STACK_SIZE and some misc debug options

The V8_STACK_SIZE config option and EMCC_V8_STACK_SIZE environment
variable can be used to control the V8 stack size, which is
especially importat on windows for large codebases.

Also adds some debug prints and options, including EMCC_DEBUG=serialize
which will still chunkify processing, but execute serially in the same
thread instead of using multiprocessing.