gdb/dwarf: make read_{loc,rng}list_index return sect_offset

gdb/dwarf: make read_{loc,rng}list_index return sect_offset

I think it's wrong that read_loclist_index and read_rnglist_index return
a CORE_ADDR.  A CORE_ADDR is an address in the program.  These functions
return offset in sections (.debug_loclists and .debug_rnglists).  I
think sect_offset is more appropriate.

I'm wondering if struct attribute should have a "set_sect_offset"
method, that takes  a sect_offset parameter, or if it's better to be
left as a simple "unsigned".

gdb/ChangeLog:

	* dwarf2/read.c (read_loclist_index, read_rnglist_index): Return
	a sect_offset.
	(read_attribute_reprocess): Adjust.

Change-Id: I0e22e0864130fb490072b41ae099762918b8ad4d