RISC-V: Support new .option arch directive.

https://github.com/riscv/riscv-asm-manual/pull/67

Format:
.option arch, +<extension><version>, ...
.option arch, -<extension>
.option arch, =<ISA string>

The new direcitve is used to enable/disable extensions for the specific
code region.  For example,

.attribute arch, "rv64ic"   # arch = rv64i2p0_c2p0
.option push
.option arch, +d2p0, -c     # arch = rv64i2p0_f2p0_d2p0, f is added implied
.option arch, =rv32gc       # arch = rv32i2p0_m2p0_a2p0_f2p0_d2p0_c2p0
.option pop                 # arch = rv64i2p0_c2p0

Note that,
1. ".option rvc/norvc" have the same behavior as ".option arch +c/-c".
2. ".option arch -i" is illegal, since we cannot remove base i extension.
3. If arch=rv64i2p0, then ".option arch, +i3p0" will update the i's version
   from 2.0 to 3.0.
4. If arch=rv64i3p0, then ".option arch, +i" will update the i's version
   from 2.0 to the default one according to the chosen isa spec.

bfd/
	* elfxx-riscv.c (riscv_add_subset): If the subset is already added,
	and the new versions are not RISCV_UNKNOWN_VERSION, then update the
	versions to the subset list.
	(riscv_copy_subset): New function.  Copy the subset from list.
	(riscv_copy_subset_list): New function.  Return the new copyed list.
	(riscv_update_subset): Updated to make .option arch directives workable.
	* elfxx-riscv.h: Updated.
gas/
	* config/tc-riscv.c (riscv_subsets): Defined as a pointer.
	(riscv_rps_as): Init the subset_list to NULL, we will set it later
	once riscv_opts_stack is created or updated.
	(struct riscv_option_stack, riscv_opts_stack): Moved forward.
	(riscv_set_arch): Updated.
	(s_riscv_option): Support new .option arch directive, to add, remove
	or update subsets for the specific code region.
	(riscv_write_out_attrs): Updated.
	* doc/c-riscv.texi: Added document for new .option arch directive.
	* testsuite/gas/riscv/option-arch-01a.d: New testcase.
	* testsuite/gas/riscv/option-arch-01b.d: Likewise.
	* testsuite/gas/riscv/option-arch-01.s: Likewise..
	* testsuite/gas/riscv/option-arch-02.d: Likewise.
	* testsuite/gas/riscv/option-arch-02.s: Likewise.
	* testsuite/gas/riscv/option-arch-fail.d: Likewise.
	* testsuite/gas/riscv/option-arch-fail.l: Likewise.
	* testsuite/gas/riscv/option-arch-fail.s: Likewise.

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