Exporting mutable globals / externref globals from `.s` files

Suppose I make a `.s` file with the following:
```
.globl myGlobal
.globaltype myGlobal, externref
myGlobal:
```
This causes trouble in many places in the code. At compile time, it crashes in `extract_metadata.py` in `get_const_expr_value` because it is surprised to see `ref.null` as the expression. But also `create_named_globals` is troublesome. Ideally we'd want to export the global itself and not the `value` of the global for mutable ones because then it would be possible to change the global from JavaScript or pass it as an import to a second wasm module.

If we use an `externref` global, there's more troubles at runtime in `relocateExports` and `updateGOT` in `library_dylink.js` because both of these methods get surprised that `typeof value.value === "object"`.

So
1. ideally, for exported mutable globals the whole global object should be exported not just the value
2. it would be nice if `externref`-valued globals were supported

I am willing to contribute code for this.