[RISCV] Add zvl65536b to the target feature list

[kito-cheng]

Summary

We have zvl65536b listed in RISCVISAInfo.cpp, but we don't have
corresponding target feature in td file.

[reames]

LGTM

llvm/lib/Target/RISCV/RISCV.td

: SubtargetFeature<"no-rvc-hints", "EnableRVCHintInstrs", "false",
                        "Disable RVC Hint Instructions.">;
 def HasRVCHints : Predicate<"Subtarget->enableRVCHintInstrs()">,
                   AssemblerPredicate<(all_of(not FeatureNoRVCHints)),
                                       "RVC Hint Instructions">;
 
 def FeatureStdExtZvl32b : SubtargetFeature<"zvl32b", "ZvlLen", "32",
                                            "'Zvl' (Minimum Vector Length) 32">;
 
-foreach i = { 6-15 } in {
+foreach i = { 6-16 } in {
   defvar I = !shl(1, i);
   def FeatureStdExtZvl#I#b :
       SubtargetFeature<"zvl"#I#"b", "ZvlLen", !cast<string>(I),
                        "'Zvl' (Minimum Vector Length) "#I,
                        [!cast<SubtargetFeature>("FeatureStdExtZvl"#!srl(I, 1)#"b")]>;
 }
 
 def FeatureStdExtZve32x