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.

Change-Id: I5d6a8ef0603de7d727585004142780a527e70496
Reviewed-on: https://go-review.googlesource.com/29085
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>