runtime: make gdb test resilient to line numbering
Don't break on line number, instead break on the actual call.
This makes the test more robust to line numbering changes in the backend.
A CL (28950) changed the generated code line numbering slightly. A MOVW
$0, R0 instruction at the start of the function changed to line
10 (because several constant zero instructions got CSEd, and one gets
picked arbitrarily). That's too fragile for a test.