RISC-V: output Autovec params explicitly in --help ...

... otherwise user has no clue what -param to actually change

gcc/ChangeLog:
	* config/riscv/riscv.opt: Add --param names
	riscv-autovec-preference and riscv-autovec-lmul

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>