8378905: RISC-V: fastdebug build fails after JDK-8377554

Reviewed-by: fyang, wenanjian

src/hotspot/cpu/riscv/riscv.ad

@@ -2274,7 +2274,7 @@ encode %{
       } else if (rtype == relocInfo::metadata_type) {
         __ mov_metadata(dst_reg, (Metadata*)con);
       } else {
-        assert(rtype == relocInfo::none, "unexpected reloc type");
+        assert(rtype == relocInfo::none || rtype == relocInfo::external_word_type, "unexpected reloc type");
         __ mv(dst_reg, $src$$constant);
       }
     }