Measure thread-local CPU usage in guest profiler

#### Feature

We record the wall-clock time when each call-stack sample was taken in the guest profiler, but the Firefox profiler format also allows us to record how much CPU time was used since the last sample.

#### Benefit

This should change the profile viewer’s output format to offer total time in various places, not just number of samples. That's generally easier for people to understand.

#### Implementation

On POSIX platforms, the clock to use is `rustix::time::clock_gettime(ClockId::ThreadCPUTime)`. I don't know how to do this on Windows.

In `GuestProfiler::sample`, the call to `self.profile.add_sample` currently passes `CpuDelta::ZERO` and should instead pass the difference between two queries of the above clock.

I'm not sure if this would work correctly for an embedder who time-slices between different guests on the same OS thread. I don't know if we can detect when the current guest is swapped out or back in without some kind of magic attached to the current wasmtime-fibers code for swapping thread-local storage.

#### Alternatives

The profiler is usable without this CPU time metric.