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>

gcc/config/riscv/riscv.opt

@@ -277,7 +277,7 @@ Always inline subword atomic operations.
 
 Enum
 Name(riscv_autovec_preference) Type(enum riscv_autovec_preference_enum)
-The RISC-V auto-vectorization preference:
+Valid arguments to -param=riscv-autovec-preference=:
 
 EnumValue
 Enum(riscv_autovec_preference) String(none) Value(NO_AUTOVEC)
@@ -294,7 +294,7 @@ Target RejectNegative Joined Enum(riscv_autovec_preference) Var(riscv_autovec_pr
 
 Enum
 Name(riscv_autovec_lmul) Type(enum riscv_autovec_lmul_enum)
-The RVV possible LMUL:
+The RVV possible LMUL (-param=riscv-autovec-lmul=):
 
 EnumValue
 Enum(riscv_autovec_lmul) String(m1) Value(RVV_M1)