pcsx2/GSdx: disable AVX for the moment on linux

I need to check carefully the consequence of ABI change. So far wx is very unhappy!

Fatal Error: Mismatch between the program and library build versions detected.
The library used 2.8 (no debug,Unicode,compiler with C++ ABI 1002,wx containers,compatible with 2.6),
and your program used 2.8 (no debug,Unicode,compiler with C++ ABI 1006,wx containers,compatible with 2.6).