Fix SConstruct for asan build

Sometimes it's easier to debug gem5 built with ASan enabled. This CL fixes
some build error when using --with-asan.

Bug: None
Test: ./scripts/build_gem5 --with-asan --with-ubsan build/ARM/gem5.debug

Change-Id: Iaaaaebc3f25749e11f97bf454ddd0153b3de56e7
Reviewed-on: https://gem5-review.googlesource.com/12511
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

src/arch/arm/isa.hh

@@ -461,7 +461,7 @@ namespace ArmISA
                     return INTREG_SP0;
                   default:
                     panic("Invalid exception level");
-                    break;
+                    return 0;  // Never happens.
                 }
             } else {
                 return flattenIntRegModeIndex(reg);

src/cpu/reg_class.hh

@@ -84,7 +84,7 @@ class RegId {
     static constexpr size_t Scale = TheISA::NumVecElemPerVecReg;
     friend struct std::hash<RegId>;
   public:
-    RegId() {};
+    RegId() : regClass(IntRegClass), regIdx(0), elemIdx(-1) {}
     RegId(RegClass reg_class, RegIndex reg_idx)
         : regClass(reg_class), regIdx(reg_idx), elemIdx(-1)
     {