Treat -g1 as -gline-tables-only

-g1 on gcc (and also IBM's xlc) are documented to be very similar to
-gline-tables-only. Our -gline-tables-only might still be more verbose than -g1
on other compilers, but currently we treat -g1 as -g, and so we're producing
much more debug info at -g1 than everybody else. Treating -g1 as
-gline-tables-only brings us much closer to what everyone else is doing.

For more information, see the discussion on
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-October/039649.html