YJIT: Make iseq_get_location consistent with iseq.c

[k0kubun]

I've felt YJIT's disasm sometimes shows None where --dump=insns would show the location just fine. It seems that YJIT uses #{iseq_path}:#{base_label} while the rest of the CRuby code uses #{label}@#{iseq_path}:#{lineno} for describing an iseq location. It seems more useful to use the common format, at least because label generally doesn't return nil unlike base_label.

Before

  # Block: /Users/k0kubun/tmp/a.rb:None (ISEQ offset: 0)
  ..
  # YJIT entry: /Users/k0kubun/tmp/a.rb:bar

After

  # Block: <main>@/Users/k0kubun/tmp/a.rb:1 (ISEQ offset: 0)
  ..
  # YJIT entry: bar@/Users/k0kubun/tmp/a.rb:5