directfb: disable for sparc

Requires atomic operations.
Fixes following autobuild failure:
http://autobuild.buildroot.net/results/8205100c166659c47daff66b1e63edc82eeb7930/



[Peter: also hide toolchain comment]
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>