Fix wasm non-web usage of env::now

[bnjbvr]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

A wasm embedder that hasn't enabled the web feature will still get the Instant::now() function generated, even if they end up using a custom now function. After all, the compiler/link don't have runtime knowledge about how the program runs, so it could be that ThreadProfiler::initialize is never called, so the default value of now_ns that makes use of Instant::now could be called at any point.

This changes the now_ns function so it's defined only using Instant for native target and wasm when web is enabled. In the other case, we assume this is never called and it's a programmer error because we don't have any time reporter. I didn't want to cause a panic! in there, so returned a dummy value instead; let me know what you think is preferred.

Related Issues

Fixes #118.