slapos[component/libffi]: Compile for generic CPU arch

/reviewed-on nexedi/slapos!361

component/libffi/buildout.cfg

@@ -16,6 +16,8 @@ patches =
   ${:_profile_base_location_}/includedir.diff#e430307d16a0d215a24ec2acff23d184
 configure-options =
   --disable-static
+  --enable-portable-binary
+  --with-gcc-arch=generic
 # on x86_64 OpenSuse, libraries are installed under parts/libffi/lib64.
 make-targets =
   install && cp -av ${:location}/lib64/* ${:location}/lib/ || true