Only use __builtin_* with GCC >= 3.4

Fix gcc 3.3 builds, broken in revision 5465.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5466 c046a42c-6fe2-441c-8c8c-71466251a162