scons: Pull some python related mechanisms out of USE_PYTHON guards.

We don't want to build certain files if USE_PYTHON is disabled, but we
can still tell scons how to.

Change-Id: I38c7c93f609cfcedc350f8270f0b239b69c4f101
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48367
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>

SConstruct

@@ -516,12 +516,6 @@ if main['USE_PYTHON']:
         if not py_version:
             error("Can't find a working Python installation")
 
-    marshal_env = main.Clone()
-
-    # Bare minimum environment that only includes python
-    marshal_env.Append(CCFLAGS='$MARSHAL_CCFLAGS_EXTRA')
-    marshal_env.Append(LINKFLAGS='$MARSHAL_LDFLAGS_EXTRA')
-
     # Found a working Python installation. Check if it meets minimum
     # requirements.
     ver_string = '.'.join(map(str, py_version))
@@ -532,6 +526,12 @@ if main['USE_PYTHON']:
         warning('Embedded python library too new. '
                 'Python 3 expected, found %s.' % ver_string)
 
+marshal_env = main.Clone()
+
+# Bare minimum environment that only includes python
+marshal_env.Append(CCFLAGS='$MARSHAL_CCFLAGS_EXTRA')
+marshal_env.Append(LINKFLAGS='$MARSHAL_LDFLAGS_EXTRA')
+
 main['HAVE_PKG_CONFIG'] = main.Detect('pkg-config')
 
 with gem5_scons.Configure(main) as conf:
@@ -704,9 +704,7 @@ Build variables for {dir}:
     env.Append(CCFLAGS='$CCFLAGS_EXTRA')
     env.Append(LINKFLAGS='$LDFLAGS_EXTRA')
 
-    exports=['env']
-    if main['USE_PYTHON']:
-        exports.append('marshal_env')
+    exports=['env', 'marshal_env']
 
     # The src/SConscript file sets up the build rules in 'env' according
     # to the configured variables.  It returns a list of environments,