Don't mark reachable extern fns as internal

If a function is marked as external, then it's likely desired for use with some
native library, so we're not really accomplishing a whole lot by internalizing
all of these symbols.
auto merge of #10539 : alexcrichton/rust/external-linkage, r=pcwalton

If a function is marked as external, then it's likely desired for use with some
native library, so we're not really accomplishing a whole lot by internalizing
all of these symbols.