[cmake] change -Os flag -> Ofast to boost performance on Android (PaddlePaddle#652)

- change -Os flag -> Ofast to boost performance on Android

Author: DefTruth

CMakeLists.txt

@@ -46,8 +46,8 @@ endif()
 
 if(ANDROID)
   # To reduce the volume of the library
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g0 -Os -ffunction-sections -fdata-sections")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g0 -Os -ffunction-sections -fdata-sections")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g0 -Ofast -ffunction-sections -fdata-sections")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g0 -Ofast -ffunction-sections -fdata-sections")
 endif()
 
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