[gdb/testsuite] Improve argument syntax of proc arange

[gdb/testsuite] Improve argument syntax of proc arange

The current syntax of proc arange is:
...
  proc arange { arange_start arange_length {comment ""} {seg_sel ""} } {
...
and a typical call looks like:
...
  arange $start $len
...

This style is somewhat annoying because if you want to specify the last
parameter, you need to give the default values of all the other optional ones
before as well:
...
  arange $start $len "" $seg_sel
...

Update the syntax to:
...
    proc arange { options arange_start arange_length } {
       parse_options {
           { comment "" }
           { seg_sel "" }
       }
...
such that a typical call looks like:
...
  arange {} $start $len
...
and a call using seg_sel looks like:
...
  arange {
    seg_sel $seg_sel
  } $start $len
...

Also update proc aranges, which already has an options argument, to use the
new proc parse_options.

Tested on x86_64-linux.

Co-Authored-By: Simon Marchi <simon.marchi@polymtl.ca>