testsuite, gdb.btrace: adjust expected source line in non-stop.exp

testsuite, gdb.btrace: adjust expected source line in non-stop.exp

In gdb.btrace/non-stop.exp, we hard-code expected source lines assuming we
know how they would match to the recorded trace.  Despite the fact that we
should really have been using an assembly source, the assumptions work
pretty well.

With clang-6 -m32, we found a case where the assumptions do not hold.
Adjust the expected source lines a little bit to cover that case, as well.

Should we run into more cases like this, we will have to switch to an
assembly source file.