version_56: Add --strip that removes debug info (#1787)

This is sort of like --strip on a native binary. The more specific use case for us is e.g. you link with a library that has -g in its CFLAGS, but you don't want debug info in your final executable (I hit this with poppler now). We can make emcc pass this to binaryen if emcc is not building an output with intended debug info.