gdb: add intern methods to objfile_per_bfd_storage
This allows keeping the objfile_per_bfd_storage implementation details
into objfile_per_bfd_storage, instead of into objfile. And this makes
the intern methods usable for code that only has an
objfile_per_bfd_storage to work with.
gdb/ChangeLog:
* objfiles.h (struct objfile_per_bfd_storage) <intern>: New
methods.
(struct objfile) <intern>: Use
objfile::objfile_per_bfd_storage::intern.