Move the SSE2/AVX/Neon/etc. flags into the compiler mkspecs

Move the SSE2/AVX/Neon/etc. flags into the compiler mkspecs

This allows us to have different flags for the compilers for
supporting the same feature. For example, the official flag in GCC to
support AVX2 is -mavx2, but ICC does not support it (yet), requiring
-march=core-avx2 or -xCORE-AVX2. That flag, instead, enables support
for all the features that the "Core-AVX2" processor (codename Haswell)
will support. And clearly, the MSVC flags are different.

Change-Id: I33b6d8617520925e807747180a8dbaafd79b7a9a
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>