[Target] Include bitset (NFC)

These files are relying on the transitive include of <bitset> from
GIMatchTableExecutor.h, which doesn't actually use std::bitset.

Author: kazutakahirata

llvm/lib/Target/ARM/ARMSubtarget.h

@@ -32,6 +32,7 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/TargetParser/Triple.h"
+#include <bitset>
 #include <memory>
 #include <string>
 

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -25,6 +25,7 @@
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/Target/TargetMachine.h"
+#include <bitset>
 
 #define GET_SUBTARGETINFO_HEADER
 #include "RISCVGenSubtargetInfo.inc"