Unable to compile Swift runtime on Linux

ProfilingATNSimulator.swift references NSEC_PER_SEC, which is undefined on Linux.

I've seen others do:
```swift
#if os(Linux)
	import let CDispatch.NSEC_PER_SEC
#endif
```

Though I'll leave it to the maintainers to decide the right approach (I didn't even know import let was a thing and neither did the GitHub syntax highlighter apparently).