Allocate compunit_symtab on heap

Allocate compunit_symtab on heap

This patch changes compunit_symtab to be allocated on the heap, using
'new'.  It also changes the container that holds these in the objfile.
I chose to use an intrusive_list to store compunit_symtab because I
think pointer stability is needed here.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33435
Approved-By: Simon Marchi <simon.marchi@efficios.com>