tile: improve simulator notification for relative paths in dlopen

tile: improve simulator notification for relative paths in dlopen

Normally, the simulator is notified of absolute pathnames by the
_dl_load_hook hook.  However, when a relative pathname is used, the
simulator may not know that the relative path matches a path that
it could figure out in the file system that it has access to.
Instead we provide a simplified version of the realpath function
so we can pass a plausible absolute pathname to the simulator.

Since we're now doing more work at object load time, we also add
a guard so we do no work at all if we're not running on the simulator.