Properly merge GNU_PROPERTY_X86_ISA_1_USED (x86_64).

gold/
	PR ld/23486
	* x86_64.cc (Target_x86_64::Target_x86_64): Initialize
	object_isa_1_used_.
	(Target_x86_64::object_isa_1_used_): New data member.
	(Target_x86_64::record_gnu_property): Save ISA_1_USED bits for object.
	(Target_x86_64::merge_gnu_properties): Merge ISA_1_USED bits.