[RISCV] Use X_X0 instead of literal 0. NFC (llvm#158753)

Author: topperc

lld/ELF/Arch/RISCV.cpp

@@ -756,7 +756,7 @@ static void relaxCall(Ctx &ctx, const InputSection &sec, size_t i, uint64_t loc,
 
   // When the caller specifies the old value of `remove`, disallow its
   // increment.
-  if (remove >= 6 && rvc && isInt<12>(displace) && rd == 0) {
+  if (remove >= 6 && rvc && isInt<12>(displace) && rd == X_X0) {
     sec.relaxAux->relocTypes[i] = R_RISCV_RVC_JUMP;
     sec.relaxAux->writes.push_back(0xa001); // c.j
     remove = 6;