[gdb/testsuite] Fix gdb.dwarf2/dw2-out-of-range-end-of-seq.exp on aarch64

[gdb/testsuite] Fix gdb.dwarf2/dw2-out-of-range-end-of-seq.exp on aarch64

On aarch64-linux, with test-case gdb.dwarf2/dw2-out-of-range-end-of-seq.exp I
run into:
...
(gdb) run ^M
Starting program: dw2-out-of-range-end-of-seq ^M
^M
Program received signal SIGILL, Illegal instruction.^M
main () at src/gdb/testsuite/gdb.dwarf2/main.c:1^M
1       /* This testcase is part of GDB, the GNU debugger.^M
(gdb) FAIL: gdb.dwarf2/dw2-out-of-range-end-of-seq.exp: runto: run to main
...

There are two problems here:
- the test-case contains a hardcoded "DW_LNS_advance_pc 1" which causes the
  breakpoint pointing in the middle of an insn
- the FAIL triggers on aarch64-linux, but not on x86_64-linux, because the
  test-case uses 'main_label' as the address of the first and only valid entry
  in the line table, and:
  - on aarch64-linux, there's no prologue, so main_label and main coincide,
    while
  - on x86_64-linux, there's a prologue, so main_label is different from main.

Fix these problems by:
- eliminating the use of "DW_LNS_advance_pc 1", and using
  "DW_LNE_set_address $main_end" instead, and
- eliminating the use of main_label, using "DW_LNE_set_address $main_start"
  instead.

Tested on both x86_64-linux and aarch64-linux.