[gdb/testsuite] Support .debug_aranges in dwarf assembly

[gdb/testsuite] Support .debug_aranges in dwarf assembly

Add a proc aranges such that we can generate .debug_aranges sections in dwarf
assembly using:
...
  cu { label cu_label } {
  ...
  }

  aranges {} cu_label {
    arange $addr $len [<comment>] [$segment_selector]
  }
...

Tested on x86_64-linux.