gdb/dwarf: make some fields of dwarf2_per_cu private

The comments on these fields mention that they should be private, but we
can't.  I think this comes from the time where dwarf2_per_cu was and had
to remain POD.  I don't think it's relevant anymore, there are other
private fields anyway.  Make them private.

Change-Id: I1915ea531f42d685f68ff547833816906f79cd58
Approved-By: Tom Tromey <tom@tromey.com>