cmake: By default optimize for current arch

Note: it requires GCC 4.7. Otherwise use -DDISABLE_ADVANCE_SIMD=OFF to restore
previous behavior. It will impact Ubuntu precise (12.04)
gsdx: enable AVX with GCC

* Use overloaded function instead of specialized template
=> see http://stackoverflow.com/questions/3052579/explicit-specialization-in-non-namespace-scope

* replace _mm256_slli_si128 by _mm256_slli_si256
  I hope they're equivalent. I didn't find any info on _mm256_slli_si128,
  however srl use _mm256_srli_si256
Merge pull request #333 from PCSX2/linux-avx

Support of AVX build for linux