Use a more sensible heuristic to detect default-target

[graydon]

Rustboot probes /System/Library to determine if it's on OSX vs. Linux. This is a ridiculous sniffing heuristic, to say the least. Write something more robust for detecting meaningful unix flavours (those that effect backend / objfile choices).

[pcwalton]

Now uses uname -s.

[pcwalton]

Marking this fixed.