cpu-o3: Replace integral constants by named constants in FU pool (#1556)

This replaces hardcoded integral values with more explicit constant
names in the code allocating functional units to instructions.

This commit follows ba5886aee7 which
should have read:

"If an instruction requires a functional unit that is not present in the
model (e.g., because it is not present in the configuration), O3CPU
treats it as a 1-cycle operation.

This commit changes the behavior to make the cpu panic when this
happens. The cpu panics only if the instruction reaches the head of the
ROB, meaning it is ok to have unsupported instructions on the wrong
path.

Thanks to Chandana S. Deshpande (deshpande.s.chandana@gmail.com) for
finding the issue."

Change-Id: I5e0a37e5fb8404cb5496bd2cb0a9a5baeae3b895

Co-authored-by: Arthur perais <arthur.perais@univ-grenoble-alpes.fr>

src/cpu/o3/fu_pool.cc

@@ -164,7 +164,7 @@ FUPool::getUnit(OpClass capability)
     //  If this pool doesn't have the specified capability,
     //  return this information to the caller
     if (!capabilityList[capability])
-        return -2;
+        return NoCapableFU;
 
     int fu_idx = fuPerCapList[capability].getFU();
     int start_idx = fu_idx;
@@ -175,7 +175,7 @@ FUPool::getUnit(OpClass capability)
         fu_idx = fuPerCapList[capability].getFU();
         if (fu_idx == start_idx) {
             // No FU available
-            return -1;
+            return NoFreeFU;
         }
     }