Fix undeclared identifier 'attr' error

Building with 'ENABLE_JIT=1, ENABLE_SYSTEM=1, and ENABLE_ELF_LOADER=0'
causes the following errors:
src/riscv.c:582:18: error: use of undeclared identifier 'attr'
  582 |     u8250_delete(attr->uart);
      |                  ^
src/riscv.c:583:17: error: use of undeclared identifier 'attr'
  583 |     plic_delete(attr->plic);
      |                 ^

These errors occur because attr is not defined under these conditions.
Fix this by adjusting the guard for attr to
(RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)), ensuring it is defined as
needed.

Author: eleanorLYJ

src/riscv.c

@@ -621,8 +621,10 @@ bool rv_has_halted(riscv_t *rv)
 void rv_delete(riscv_t *rv)
 {
     assert(rv);
-#if !RV32_HAS(JIT)
+#if !RV32_HAS(JIT) || (RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER))
     vm_attr_t *attr = PRIV(rv);
+#endif
+#if !RV32_HAS(JIT)
     map_delete(attr->fd_map);
     memory_delete(attr->mem);
     block_map_destroy(rv);