The Mali-400 is a very powerful GPU but not very easy to use.
This series of article will help you understand and use it.
First of all download « lite edition » of Sourcery from Mentor: GCC Toolchain
This includes a gcc compiler and OpenCl (support): Mali OpenCl
Be careful that the OpenCL and Gcc toolchain are 32 bits. If you use a 64bits version, be sure to install 32bits library.
To verify is everything is correctly installed type
cd Mali_OpenCL_SDK_v1.1.0/samples/sobel and type make
Depending on your environment it can be difficult to install a cross compilation, if you can not succeed you can compile directly on the Olinuxino A20, but compilation time will be longer.