MacOS: Update vm exit handling to be on par with Linux

[jounathaen]

On Linux, all other VM threads are ended, once a single one has completed execution. This solves Uhyve hanging on failures of hermit.

uhyve/src/linux/mod.rs

Lines 134 to 141 in 06d18a1

	// Wait for one vCPU to return with an exit code.
	barrier.wait();
	for thread in &threads {
	KickSignal::pthread_kill(thread.as_pthread_t()).unwrap();
	}

The macOS code should be updated to be on-par with the Linux version.