Use "new" to allocate gdb_bfd_data

Use "new" to allocate gdb_bfd_data

This changes gdb_bfd_data to be allocated with new and destroyed with
delete.

ChangeLog
2017-10-20  Tom Tromey  <tom@tromey.com>

	* gdb_bfd.c (gdb_bfd_ref): Use new.
	(struct gdb_bfd_data): Add constructor, destructor, and member
	initializers.
	(gdb_bfd_unref): Use delete.