arch-riscv: Fix bug in serialize and unserialize of Interrutps

arch-riscv: Fix bug in serialize and unserialize of Interrutps

When serialize and unserialize an variable, the parameters passed to
SERIALIZE_SCALAR() and UNSERIALIZE_SCALAR() must be the same and should be a
general variable name. If not, the expected item would not be found with
UNSERIALIZE_SCALAR() and a fatal error would be introduced.
This patch fix the bug in class Interrupts of RISCV.

Change-Id: I7dd7ab6805651149304959bdf7ee9f3be9d9eaff
Signed-off-by: Ian Jiang <ianjiang.ict@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22643
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Maintainer: Alec Roelke <alec.roelke@gmail.com>