RISC-V: Tidy riscv assembler and disassembler.

RISC-V: Tidy riscv assembler and disassembler.

Tidy the gas/config/tc-riscv.c and opcodes/riscv-dis.c, to prepare for
moving the released extensions (including released vendor extensions)
from integration branch back to mainline.

* Added parts of missing comments.

* Updated md_show_usage.

* For validate_riscv_insn, riscv_ip and print_insn_args, unify the
  following pointer names,
  - oparg: pointed to the parsed operand defined in the riscv_opcodes.
  - asarg: pointed to the parsed operand from assembly.
  - opargStart: recorded the parsed operand name from riscv_opcodes.
  - asargStart: recorded the parsed operand name from assembly.

gas/
	* config/tc-riscv.c: Added parts of missind comments and updated
	the md_show_usage.
	(riscv_multi_subset_supports): Tidy codes.
	(validate_riscv_insn): Unify the pointer names, oparg, asarg,
	opargStart and asargStart, to prepare for moving the released
	extensions from integration branch back to mainline.
	(riscv_ip): Likewise.
	(macro_build): Added fmtStart, also used to prepare for moving
	released extensions.
	(md_show_usage): Added missing descriptions for new options.
opcodes/
	* riscv-dis.c (print_insn_args): Unify the pointer names,
	oparg and opargStart, to prepare for moving the released
	extensions from integration branch back to mainline.