[binutils-gdb] Use gdb_bfd_ref_ptr in target_bfd

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT ade72a3453670d518ba14fe47bc7a64df81ce766 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: ade72a3453670d518ba14fe47bc7a64df81ce766

Use gdb_bfd_ref_ptr in target_bfd

I noticed that target_bfd was using manual reference counting for the
BFD it held.  This patch changes it to use gdb_bfd_ref_ptr instead.

Tested by the buildbot.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* bfd-target.c (target_bfd::m_bfd): Now a gdb_bfd_ref_ptr.
	(target_bfd::target_bfd, target_bfd::~target_bfd): Update.