misc: Stop excluding code when building the NULL ISA.

The BaseCPU needs a little extra hacking because it tries to create
default objects based on what the ISA is. If the ISA isn't recognized,
then the types will be set to None, and some extra checks have been
added as the type is set up.

Change-Id: Ia3cae313e1a96a953d2316d9192f41a8fd28c141
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62195
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -576,7 +576,6 @@ def makeTheISA(source, target, env):
 #ifndef __CONFIG_THE_ISA_HH__
 #define __CONFIG_THE_ISA_HH__
 
-#define IS_NULL_ISA ${{is_null_isa}}
 #define TheISA ${{namespace(target_isa)}}
 
 #endif // __CONFIG_THE_ISA_HH__''')