RISC-V: Clarify the behavior of .option arch directive.

* To be consistent with -march option, removed the "=" operator when
user want to reset the whole architecture string.  So the formats are,

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

* Don't allow to add or remove the base extensions in the .option arch
directive.  Instead, users should reset the whole architecture string
while they want to change the base extension.

* The operator "+" won't update the version of extension, if the
extension is already in the subset list.

bfd/
	* elfxx-riscv.c (riscv_add_subset): Don't update the version
	if the extension is already in the subset list.
	(riscv_update_subset): To be consistent with -march option,
	removed the "=" operator when user want to reset the whole
	architecture string.  Besides, Don't allow to add or remove
	the base extensions in the .option arch directive.
gas/
	* testsuite/gas/riscv/option-arch-01.s: Updated since we cannot
	add or remove the base extensions in the .option arch directive.
	* testsuite/gas/riscv/option-arch-02.s: Likewise.
	* testsuite/gas/riscv/option-arch-fail.l: Likewise.
	* testsuite/gas/riscv/option-arch-fail.s: Likewise.
	* testsuite/gas/riscv/option-arch-01a.d: Set -misa-spec=2.2.
	* testsuite/gas/riscv/option-arch-01b.d: Likewise.
	* testsuite/gas/riscv/option-arch-02.d: Updated since the .option
	arch, + won't change the version of extension, if the extension is
	already in the subset list.
	* testsuite/gas/riscv/option-arch-03.s: Removed the "=" operator
	when resetting the whole architecture string.

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