Remove `_` prefix/name mangling for native symbols

With the original JS compiler native symbols historically got prefixed with an `_`.

With fastcomp this tradition was continued with `JSBackend.cpp` adding extra `_` to all its exports.

With the llvm backend we still continue this mangling but only in the JS wrapper code. The actual exports from the wam module do not have this mangling but the wrapper code added this. See:

https://github.com/emscripten-core/emscripten/blob/119980fe4839b943599edf5cc222131db41c22ff/tools/shared.py#L736

and also the runtime equivalent:

https://github.com/emscripten-core/emscripten/blob/119980fe4839b943599edf5cc222131db41c22ff/src/jsifier.js#L27
https://github.com/emscripten-core/emscripten/blob/119980fe4839b943599edf5cc222131db41c22ff/src/jsifier.js#L32

We should try to phase this out over time.

I suggest we add a new setting called `MANGLE_NATIVE_SYMBOLS` which defaults to 1. We can then default this to `0 in STRICT mode and one day change the default for everyone.

Alternative names for this setting:

- `MANGLE_EXPORTS`
- `MANGLED_EXPORTS`
- `PREFIX_EXPORTS`

When this setting is enabled, EXPORTED_FUNCTIONS should also not contain the mangling so users would have to move from `EXPORTED_FUNCTIONS=[_main]` to `EXPORTED_FUNCTIONS=[main]`