gdb: move sect_offset and cu_offset to dwarf2/types.h

gdb: move sect_offset and cu_offset to dwarf2/types.h

I want to move the call_site stuff out of gdbtypes.h, to a new header
file, to break some cyclic include problem.  The call_site stuff uses
cu_offset, also defined in gdbtypes.h, so cu_offset also needs to move
somewhere else (otherwise, call-site.h will need to include gdbtypes.h,
and we are back to square 1).  I could move cu_offset to the future new
file dwarf2/call-site.h, but it doesn't sound like a good place for it,
at cu_offset is not specific to call sites, it's used throughout
dwarf2/.  So, move it to its own file, dwarf2/types.h.  For now,
gdbtypes.h includes dwarf2/types.h, but that will be removed once the
call site stuff is moved to its own file.

Move sect_offset with it too.  sect_offset is not a DWARF-specific
concept, but for the moment it is only used in dwarf2/.

Change-Id: I1fd2a3b7b67dee789c4874244b044bde7db43d8e
Reviewed-By: Bruno Larsen <blarsen@redhat.com>