RISC-V: Add new option -mrelax/-mno-relax.

RISC-V: Add new option -mrelax/-mno-relax.

	gas/
	* config/tc-riscv.c (options): Add OPTION_RELAX and
	OPTION_NO_RELAX.
	(md_longopts): New option -mrelax and -mno-relax.
	(md_parse_option): Handle -mrelax and -mno-relax.
	* doc/c-riscv.texi: Document for -mrelax and -mno-relax.
	* testsuite/gas/riscv/no-relax-reloc.d: New.
	* testsuite/gas/riscv/no-relax-reloc.s: New.
	* testsuite/gas/riscv/relax-reloc.d: New.
	* testsuite/gas/riscv/relax-reloc.s: New.