gdb/testsuite/dwarf: convert _section proc to use parse_options

New in v2: return early if the section is .debug_str

The following patch will add more options to the _section proc, so
convert it to use options, to be more user-friendly.

Change-Id: I971e4e10e55d63af2a5e29dc5e1d00f368b3ecaa
Approved-By: Andrew Burgess <aburgess@redhat.com>