Add -z bndplt support (for Intel MPX).
    
    gold/
    	PR gold/17643
    	* options.h (-z bndplt): New option.
    	* x86_64.cc (Output_data_plt_x86_64::regular_count): New method.
    	(Output_data_plt_x86_64::address_for_global): Move implementation into
    	virtual method.
    	(Output_data_plt_x86_64::address_for_local): Likewise.
    	(Output_data_plt_x86_64::got): New method.
    	(Output_data_plt_x86_64::got_plt): New method.
    	(Output_data_plt_x86_64::got_irelative): New method.
    	(Output_data_plt_x86_64::do_address_for_global): New virtual method.
    	(Output_data_plt_x86_64::do_address_for_local): New virtual method.
    	(class Output_data_plt_x86_64_bnd): New class.
    	(Target_x86_64::do_make_data_plt): Move out of line and specialize
    	for each size (both overloads).
    	(Output_data_plt_x86_64::set_final_data_size): Cosmetic changes.
    	* testsuite/Makefile.am (bnd_plt_1): New test case.
    	(bnd_ifunc_1): New test case.
    	(bnd_ifunc_2): New test case.
    	* testsuite/Makefile.in: Regenerate.
    	* testsuite/bnd_ifunc_1.s: New source file.
    	* testsuite/bnd_ifunc_1.sh: New shell script.
    	* testsuite/bnd_ifunc_2.s: New source file.
    	* testsuite/bnd_ifunc_2.sh: New shell script.
    	* testsuite/bnd_plt_1.s: New source file.
    	* testsuite/bnd_plt_1.sh: New shell script.