scons: Allow sources and libs called multiple times

In Python, "+=" operator for list acts more like append and list assign
doesn't make a copy. This will cause unexpected append to the orignal
list. Since we have multiple env to "declare", these functions will be
called multiple times and could wrongly append duplicated entries in
later calls.

Make a copy before appending the entries from filter to avoid this
problem.

Change-Id: I144d5054e4d93191ebc94b93291ff9a3f8a6c429
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58409
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -38,6 +38,7 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import collections
+import copy
 import distutils.spawn
 import itertools
 import os
@@ -329,13 +330,13 @@ class TopLevelBase(object, metaclass=TopLevelMeta):
         self.dir = Dir('.')
 
     def sources(self, env):
-        srcs = self.srcs
+        srcs = copy.copy(self.srcs)
         for f in self.filters:
             srcs += Source.all.apply_filter(env, f)
         return srcs
 
     def libs(self, env):
-        libs = self.sourceLibs
+        libs = copy.copy(self.sourceLibs)
         for f in self.filters:
             libs += SourceLib.all.apply_filter(env, f)
         return libs