[gdb/testsuite] Fix gdb.dwarf2/dw2-lines.exp for m32 pie

As reported in PR29043, when running test-case gdb.dwarf2/dw2-lines.exp with
target board unix/-m32/-fPIE/-pie, we run into:
...
Breakpoint 2, 0x56555540 in bar ()^M
(gdb) PASS: gdb.dwarf2/dw2-lines.exp: cv=2: cdw=32: lv=2: ldw=32: \
  continue to breakpoint: foo \(1\)
next^M
Single stepping until exit from function bar,^M
which has no line number information.^M
0x56555587 in main ()^M
(gdb) FAIL: gdb.dwarf2/dw2-lines.exp: cv=2: cdw=32: lv=2: ldw=32: \
  next to foo (2)
...

The problem is that the bar breakpoint ends up at an unexpected location
because:
- the synthetic debug info is incomplete and doesn't provide line info
  for the prologue part of the function, so consequently gdb uses the i386
  port prologue skipper to get past the prologue
- the i386 port prologue skipper doesn't get past a get_pc_thunk call.

Work around this in the test-case by breaking on bar_label instead.

Tested on x86_64-linux with target boards unix, unix/-m32, unix/-fPIE/-pie and
unix/-m32/-fPIE/-pie.

gdb/testsuite/gdb.dwarf2/dw2-lines.exp

@@ -114,7 +114,7 @@ proc test_1 { _cv _cdw64 _lv _ldw64 {_string_form ""}} {
 	return -1
     }
 
-    gdb_breakpoint "bar"
+    gdb_breakpoint "bar_label"
     gdb_continue_to_breakpoint "foo \\(1\\)"
 
     gdb_test "next" "foo \\(2\\).*" "next to foo (2)"