Add detection of AVX and AVX2 support in the compiler.

Add detection of AVX and AVX2 support in the compiler.

Intel CC 12.1 supports AVX2 but only with -march=core-avx2. The -mavx2
option produces a warning.

GCC 4.6 does not recognise any option.

GCC 4.7 recognises both -mavx2 and -march=core-avx2 so let's use the
latter for now. We may need to change to -mavx2 when there's an AMD
processor that supports AVX2 too.

Change-Id: I529240e6e6c2c0e3942d357e0320212d954fe4de
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>