SCons: Make the ISA parser a source for its output files like the comments say.

There was a change a while ago that refactored some scons stuff which got rid
of cpu_models.py but also accidentally got rid of the ISA parser as a source
for its target files. That meant that changes which affected the parser
wouldn't cause a rebuild unless they also changed one of the description
files. This change fixes that.

src/arch/SConscript

@@ -89,6 +89,8 @@ env.Append(SCANNERS = isa_scanner)
 # output from the ISA description (*.isa) files.
 #
 
+isa_parser = File('isa_parser.py')
+
 # The emitter patches up the sources & targets to include the
 # autogenerated files as targets and isa parser itself as a source.
 def isa_desc_emitter(target, source, env):
@@ -102,7 +104,12 @@ def isa_desc_emitter(target, source, env):
     # We also get an execute file for each selected CPU model.
     target += [CpuModel.dict[cpu].filename for cpu in cpu_models]
 
-    return target, source + [ Value(m) for m in cpu_models ]
+    # List the isa parser as a source.
+    source += [ isa_parser ]
+    # Add in the CPU models.
+    source += [ Value(m) for m in cpu_models ]
+
+    return target, source
 
 ARCH_DIR = Dir('.')
 
@@ -114,7 +121,7 @@ def isa_desc_action_func(target, source, env):
     sys.path[0:0] = [ ARCH_DIR.srcnode().abspath ]
     import isa_parser
 
-    models = [ s.get_contents() for s in source[1:] ]
+    models = [ s.get_contents() for s in source[2:] ]
     cpu_models = [CpuModel.dict[cpu] for cpu in models]
     parser = isa_parser.ISAParser(target[0].dir.abspath, cpu_models)
     parser.parse_isa_desc(source[0].abspath)