UserspaceEmulator: Implement the execve() syscall :^)

This virtual syscall works by exec'ing the UserspaceEmulator itself,
with the emulated program's provided arguments as the arguments to the
new UserspaceEmulator instance.

This means that we "follow" exec'ed programs and emulate them as well.
In the future we might want to make this an opt-in (or opt-out, idk)
behavior, but for now it's what we do.

This is really quite cool, I think! :^)

DevTools/UserspaceEmulator/Emulator.h

@@ -71,6 +71,7 @@ private:
     void setup_stack(const Vector<String>& arguments, const Vector<String>& environment);
 
     int virt$fork();
+    int virt$execve(FlatPtr);
     int virt$get_dir_entries(int fd, FlatPtr buffer, ssize_t);
     int virt$ioctl(int fd, unsigned, FlatPtr);
     int virt$usleep(useconds_t);