gdb/dwarf2: move some things to read.h

The following 2 patches move .gdb_index and .debug_names reading code to
their own file.  Prepare this by exposing some things used by that code
to read.h.

Change-Id: If8ef135758a2ff0ab3b765cc92596da8189f3bbd
Approved-By: Tom Tromey <tom@tromey.com>