gdb: remove symtab::blockvector

symtab::blockvector is a wrapper around compunit_symtab::blockvector.
It is a bit misleadnig, as it gives the impression that a symtab has a
blockvector.  Remove it, change all users to fetch the blockvector
through the compunit instead.

Change-Id: Ibd062cd7926112a60d52899dff9224591cbdeebf
14 files changed