fix a bug when selecting which architectures to build externals for

Change-Id: I17cf17f4b90d7c73845c9b1752af114f2a59a1ab
diff --git a/aos/build/build.py b/aos/build/build.py
index acca25f..77edd89 100755
--- a/aos/build/build.py
+++ b/aos/build/build.py
@@ -661,7 +661,8 @@
       if platforms & frc_platforms:
         to_download.add(architecture + '_frc')
 
-      if platforms & (self.platforms() - pie_sanitizers - frc_platforms):
+      if platforms & (self.select_platforms(architecture=architecture) -
+                      pie_sanitizers - frc_platforms):
         to_download.add(architecture)
 
     for download_target in to_download: