RISC-V: Don't assume the priv attributes are in order when handling them.

There is no guarantee that the priv attributes should be defined in order.
Therefore, we shouldn't have the order assumption when handling them in the
riscv_merge_attributes.  Set priv_attrs_merged to TRUE if we have handled
all of the priv attributes.

	bfd/
	* elfnn-riscv.c (riscv_merge_attributes): Once we meet one of the
	priv attributes, we will check the conflicts for all of them (major,
	minor and revision), and then set the priv_attrs_merged to TRUE to
	indicate that we have handled all of the priv attributes.  Remove
	the unused boolean priv_may_conflict, in_priv_zero and out_priv_zero.