[gdb/testsuite] Make gdb.dwarf2/dw2-line-number-zero.exp more robust

On aarch64, there are FAILs for gdb.dwarf2/dw2-line-number-zero.exp due to
problems in the prologue analyzer (filed as PR26310).

Make the test-case more robust by avoiding to use the prologue analyzer:
...
-gdb_breakpoint "bar1"
+gdb_breakpoint "$srcfile:27"
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-29  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-line-number-zero.exp: Set breakpoints on lines
	rather than function name.