RISC-V: Generate ELF priv attributes if priv instruction are explicited used.

We should generate the ELF priv attributes only if,

1. The priv attributes are already set in the assembly file.
2. The CSR are explicited used.
3. The privileged instruction are explicited used.

* There are four privileged instruction defined in the v1.11 priv spec:
`mret`, `sret`, `wfi` and `sfence.vma`.

* `sfence.vm` is dropped in the v1.10 priv spec.

* `uret` is actually a N-ext instruction.  So it is better to regard it as
an user instruction rather than the priv instruction.

* `hret` is used to return from traps in H-mode.  H-mode is removed since
the v1.10 priv spec, but probably be added in the new hypervisor spec.
Therefore, `hret` should be controlled by the hypervisor spec rather than
priv spec in the future.

* `dret` is a debug instruction.  We should record the debug spec versions
once it is explicited used in the future.

	gas/
	* config/tc-riscv.c (explicit_priv_attr): Rename explicit_csr to
	explicit_priv_attr.  It used to indicate CSR or priv instructions are
	explictly used.
	(riscv_is_priv_insn): Return True if it is a privileged instruction.
	(riscv_ip): Call riscv_is_priv_insn to check whether the instruction
	is privileged or not.  If it is, then set explicit_priv_attr to TRUE.
	(riscv_write_out_attrs): Clarification of when to generate the elf
	priv spec attributes.

	* testsuite/gas/riscv/attribute-11.s: Add comments.
	* testsuite/gas/riscv/attribute-14.s: New testcase.  Use symbol
	`priv_insn_<n>` to decide which priv instruction is expected to used.
	(<n> is a to g.)
	* testsuite/gas/riscv/attribute-14a.d: Likewise.
	* testsuite/gas/riscv/attribute-14b.d: Likewise.
	* testsuite/gas/riscv/attribute-14c.d: Likewise.
	* testsuite/gas/riscv/attribute-14d.d: Likewise.
	* testsuite/gas/riscv/attribute-14e.d: Likewise.