dg-build in lib/dg.exp mishandles line-numbers

It should either (preferrably) replace the incoming line-number
(in [lindex $args 0]) with "" (matching any line) or re-format
it with dg-format-linenum, like dg-error does.  Right now it
matches e.g. 3 with 13 which is not right in any case.

Since dg-build does not accept a line-number in the test, I
don't think it should look at it at all.  See also
<URL:http://gcc.gnu.org/ml/gcc-patches/2002-10/msg01640.html>,
references and follow-ups.

brgds, H-P