configure: loongson disable expensive optimizations in gcc O3 optimization

With gcc-4.9.2 loongson faild in test fate-dca, this is caused by option
-fexpensive-optimizations in -O3 optimization. We disable it temporarily
before the bug been fixed up.

Signed-off-by: ZhouXiaoyong <zhouxiaoyong@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>