enable fp-elim when debug info is disabled

This can almost be fully disabled, as it no longer breaks retrieving a
backtrace on OS X as verified by @alexcrichton. However, it still
breaks retrieving the values of parameters. This should be fixable in
the future via a proper location list...

Closes #7477

src/test/debug-info/function-prologue-stepping-no-split-stack.rs

@@ -244,6 +244,3 @@ fn main() {
     while_expr(40, 41, 42);
     loop_expr(43, 44, 45);
 }
-
-
-