Missing features for testing JS/Wasm backends

For testing JS/Wasm or other cross backends, nofib seems to be lacking some functionalities at the moment:

• There should be a way to specify a cross emulator to run the output executable.
• There should be a way to specify a post-linking optimizer to run on the output executable, before actually running the test.
• The overall executable size should also be measured, in addition to the main module's object size. When people deploy the JS/Wasm backend's output, it's the complete bundle size that they care about.