RISC-V: Clarify the behavior of .option rvc or norvc.

RISC-V: Clarify the behavior of .option rvc or norvc.

Add/Remove the rvc extension to/from the riscv_subsets once the
.option rvc/norvc is set.  So that we don't need to always check
the riscv_opts.rvc in the riscv_subset_supports, just call the
riscv_lookup_subset to search the subset list is enough.

Besides, we will need to dump the instructions according to the
elf architecture attributes.  That means the dis-assembler needs
to parse the architecture string from the elf attribute before
dumping any instructions, and also needs to recognized the
INSN_CLASS* classes from riscv_opcodes.  Therefore, I suppose
some functions will need to be moved from gas/config/tc-riscv.c
to bfd/elfxx-riscv.c, including riscv_multi_subset_supports and
riscv_subset_supports.  This is one of the reasons why we need
this patch.

This patch passes the gcc/binutils regressions of rv32emc-elf,
rv32i-elf, rv64gc-elf and rv64gc-linux toolchains.

bfd/
	* elfxx-riscv.c (riscv_remove_subset): Remove the extension
	from the subset list.
	(riscv_update_subset): Add/Remove an extension to/from the
	subset list.  This is used for the .option rvc or norvc.
	* elfxx-riscv.h: Added the extern bool riscv_update_subset.
gas/
	* config/tc-riscv.c (riscv_set_options): Removed the unused
	rve flag.
	(riscv_opts): Likewise.
	(riscv_set_rve): Removed.
	(riscv_subset_supports): Removed the riscv_opts.rvc check.
	(riscv_set_arch): Don't need to call riscv_set_rve.
	(reg_lookup_internal): Call riscv_subset_supports to check
	whether the rve is supported.
	(s_riscv_option): Add/Remove the rvc extension to/from the
	subset list once the .option rvc/norvc is set.