arch-x86: Mark a variable as [[maybe_unused]].

This variable is used to hold a new value for the FSW register, and is
only actually installed if the flag version of the PremFp microop is
used. Mark it as [[maybe_unused]] so clang doesn't complain.

Change-Id: Ied0e267a1b89884b369cc5f7f043c96ae86d973b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57171
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/isa/microops/fpop.isa

@@ -368,7 +368,7 @@ let {{
 
     class PremFp(Fp3Op):
         code = '''
-            RegVal new_fsw = FSW;
+            [[maybe_unused]] RegVal new_fsw = FSW;
             int src1_exp;
             int src2_exp;
             std::frexp(FpSrcReg1, &src1_exp);