gdb/dwarf: pass section to create_dwp_hash_table

Instead of passing a boolean to create_dwp_hash_table to select the
section to read, it's simpler to just pass the section.

Change-Id: Ie043c31e80518239f6403288dcf03f7769c58e8c
Approved-By: Tom Tromey <tom@tromey.com>