Don't always modify the symbol table in rlibs

Turns out that one some platforms the ar/ranlib tool will die with an assertion
if the file being added doesn't actually have any symbols (or if it's just not
an object file presumably).

This functionality is already all exercised on the bots, it just turns out that
the bots don't have an ar tool which dies in this situation, so it's difficult
for me to add a test.

Closes #10907
auto merge of #10908 : alexcrichton/rust/issue-10907, r=cmr

Turns out that one some platforms the ar/ranlib tool will die with an assertion
if the file being added doesn't actually have any symbols (or if it's just not
an object file presumably).

This functionality is already all exercised on the bots, it just turns out that
the bots don't have an ar tool which dies in this situation, so it's difficult
for me to add a test.

Closes #10907