Linux support
On Linux mmap a second copy of the shared library

I was originally planning to just use the one we have already in memory,
but Linux actually makes section headers execute only, so I couldn't even look
at that without too much effort. Secondly, I'm not sure if the debug info is even
being mapped into memory at that address since it's usually not needed and I don't
think any other library could access it there. Instead, I'm just mmaping a second
copy. This of course slightly adds to the page table pressure, but other than that it
shouldn't have any other overhead and should be no worse than having separate
debug info in any case.