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>

configure

@@ -4000,13 +4000,13 @@ elif enabled mips; then
             disable aligned_stack
             case $cpu in
                 loongson3*)
-                    cpuflags="-march=loongson3a -mhard-float"
+                    cpuflags="-march=loongson3a -mhard-float -fno-expensive-optimizations"
                 ;;
                 loongson2e)
-                    cpuflags="-march=loongson2e -mhard-float"
+                    cpuflags="-march=loongson2e -mhard-float -fno-expensive-optimizations"
                 ;;
                 loongson2f)
-                    cpuflags="-march=loongson2f -mhard-float"
+                    cpuflags="-march=loongson2f -mhard-float -fno-expensive-optimizations"
                 ;;
             esac
         ;;