Update RISC-V documentation and make sure that it is included in the gas info file.

Update RISC-V documentation and make sure that it is included in the gas info file.

	* Makefile.am (CPU_DOCS): Add c-riscv.texi.
	* Makefile.in: Regenerate.
	* doc/all.texi: Set RISCV.
	* doc/as.texinfo: Add RISCV options.
	Add RISC-V-Dependent node.
	Include c-riscv.texi.
	* doc/c-riscv.texi: Rename RISC-V Options to RISC-V-Opts.