gdb/testsuite: use gdb_get_line_number

Replaces a hard coded line number with a use of gdb_get_line_number.

I suspect that the line number has, over time, come adrift from where
it was supposed to be stopping.  When the test was first added, line
770 pointed at the final 'return 0' in function main.  Over time, as
things have been added, line 770 now points at some random location in
the middle of main.

So, I've marked the 'return 0' with a comment, and now the test will
always stop there.

I also removed an old comment from 1997 talking about how these tests
will only pass with the HP compiler, followed by an additional comment
from 2000 saying that the tests now pass with GCC.

I get the same results before and after this change.