gold: Remove BND from 64-bit x86-64 IBT PLT

gold: Remove BND from 64-bit x86-64 IBT PLT

Since MPX support has been removed from x86-64 psABI, remove BND from
64-bit IBT PLT by using 32-bit IBT PLT.

	PR gold/29851
	* x86_64.cc (Output_data_plt_x86_64_ibt<32>::first_plt_entry):
	Renamed to ...
	(Output_data_plt_x86_64_ibt<size>::first_plt_entry): This.
	(Output_data_plt_x86_64_ibt<64>::first_plt_entry): Removed.
	(Output_data_plt_x86_64_ibt<size>::do_fill_first_plt_entry):
	Drop the size == 32 check.
	(Output_data_plt_x86_64_ibt<32>::plt_entry): Renamed to ...
	(Output_data_plt_x86_64_ibt<size>::plt_entry): This.
	(Output_data_plt_x86_64_ibt<64>::plt_entry): Removed.
	(Output_data_plt_x86_64_ibt<32>::aplt_entry): Renamed to ...
	(Output_data_plt_x86_64_ibt<size>::aplt_entry): This.
	(Output_data_plt_x86_64_ibt<64>::aplt_entry): Removed.
	(Output_data_plt_x86_64_ibt<size>::do_fill_plt_entry): Drop the
	size == 32 check.
	(Output_data_plt_x86_64_ibt<size>::fill_aplt_entry): Likewise.