[RISCV] Pass FeatureBitset by reference rather than by value. NFCI

FeatureBitset is 4 64-bit values in an array. It's better passed by
reference rather than copying it.

I may be adding FeatureBitset as an argument to another function
and noticed this while working on that.

Author: topperc

llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h

@@ -257,7 +257,7 @@ struct SysReg {
   FeatureBitset FeaturesRequired;
   bool isRV32Only;
 
-  bool haveRequiredFeatures(FeatureBitset ActiveFeatures) const {
+  bool haveRequiredFeatures(const FeatureBitset &ActiveFeatures) const {
     // Not in 32-bit mode.
     if (isRV32Only && ActiveFeatures[RISCV::Feature64Bit])
       return false;