`wasm-encoder` relocation data generation

[daxpedda]

This is a feature request to add an equivalent for wat2wasm --relocatable to wasm-tools for feature parity.

I'm still stuck with wabt to be able to generate Wasm object files from WAT to link against.

[alexcrichton]

You might be interested in WebAssembly/tool-conventions#258 which has some related discussion to this. Otherwise though personally I'm not familiar enough with --relocatable to say much about it. My impression is that this would ideally be a text-format-based annotation to avoid the need to add flags to the text->binary transform (which ideally is a function without parameters other than the text IMO).