[RISCV][NFC] Use raw_svector_ostream to construct key of SubtargetMap (#72964)

To simplify some code.

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -213,13 +213,8 @@ RISCVTargetMachine::getSubtargetImpl(const Function &F) const {
       llvm::bit_floor((RVVBitsMax < 64 || RVVBitsMax > 65536) ? 0 : RVVBitsMax);
 
   SmallString<512> Key;
-  Key += "RVVMin";
-  Key += std::to_string(RVVBitsMin);
-  Key += "RVVMax";
-  Key += std::to_string(RVVBitsMax);
-  Key += CPU;
-  Key += TuneCPU;
-  Key += FS;
+  raw_svector_ostream(Key) << "RVVMin" << RVVBitsMin << "RVVMax" << RVVBitsMax
+                           << CPU << TuneCPU << FS;
   auto &I = SubtargetMap[Key];
   if (!I) {
     // This needs to be done before we create a new subtarget since any