move the entry point info into the per-bfd object

This moves the entry point information into the per-BFD object and
arranges not to recompute it when it has already been computed.

2014-01-15  Tom Tromey  <tromey@redhat.com>

	* symfile.c (init_entry_point_info): Use new "initialized" field.
	Update.
	* objfiles.h (struct entry_point) <initialized>: New field.
	(struct objfile_per_bfd_storage) <ei>: New field, moved from...
	(struct objfile) <ei>: ...here.  Remove.
	* objfiles.c (entry_point_address_query): Update.