Resolve the warning when running the project

[eeeXun]

When running sbt test. I get the warning.

[warn] .../ca2023-lab3/src/main/scala/riscv/core/WriteBack.scala:24:25: method apply in object MuxLookup is deprecated (since Chisel 3.6): Use MuxLookup(key, default)(mapping) instead
[warn]   io.regs_write_data := MuxLookup(
[warn]                         ^
[warn] one warning found

In Chisel 3.5, the syntax of MuxLookup is

MuxLookup(idx, default,
    Array(0.U -> a, 1.U -> b))

In Chisel 3.6, the syntax of MuxLookup is

MuxLookup(idx, default)(Seq(0.U -> a, 1.U -> b))

Since project is build on Chisel 3.6, we have these warnings.
This patch would not affect any code behavior. It just resolves the warning when running sbt.

[jserv]

I tend to merge this pull request at a later time since I am currently in the process of incorporating CLINT (interrupt controller) and minimal CSR support.

@eeeXun, Once the related commits are added to the main branch, please rebase your work, and I will merge accordingly.