component/libffi: move the x86_64 specific part in a conditional part

See "Conditional configuration sections" of buildout documentation.
http://www.buildout.org/en/latest/topics/variables-extending-and-substitutions.html#conditional-configuration-sections

component/libffi/buildout.cfg

@@ -11,14 +11,15 @@ shared = true
 version = 3.2.1
 url = ftp://sourceware.org/pub/libffi/libffi-${:version}.tar.gz
 md5sum = 83b89587607e3eb65c70d361f13bab43
-location = @@LOCATION@@
 patch-options = -p1
 patches =
   ${:_profile_base_location_}/includedir.diff#e430307d16a0d215a24ec2acff23d184
 configure-options =
   --disable-static
-# on x86_64 OpenSuse, libraries are installed under parts/libffi/lib64.
-make-targets =
-  install && cp -av ${:location}/lib64/* ${:location}/lib/ || true
 environment =
   PATH=${patch:location}/bin:%(PATH)s
+
+[libffi:bits64]
+# on x86_64 OpenSuse, libraries are installed under parts/libffi/lib64.
+make-targets =
+  install && cp -av @@LOCATION@@/lib64/* @@LOCATION@@/lib/ || true