Update and clean up RISC-V gas documentation.

Update and clean up RISC-V gas documentation.

	gas/
	* doc/as.texinfo (RISC-V): Alphabetize RISC-V entries.  Change
	RISC-V-Opts to RISC-V-Options.  Delete redundant space.  Add -fpic
	and related options to option list.
	* doc/c-riscv.texi: (RISC-V-Options): Renamed from RISC-V-Opts.
	(RISC-V Options): Renamed from Options.  Add missing period.
	(-fpic): Also mention -fPIC.
	(RISC-V Directives): New node.