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change minimum required hardware for gcc 6.2

Marco Clemencic requested to merge clemenci/Gaudi:LBCORE-1227.enable-sse4.2-for-gcc62 into master

To be able to profit from vectorized code, we need to enable instructions for larger vector units.

SSE4.2 instructions can be safely enabled not reducing the portability of binaries (LHCb would loose ~7% of CEs, all very old), but keeping the settings for gcc 4.9 will make the old machines still usable.

Although the new settings are not really improving the performances of current code, not using them will prevent exploiting the larger vector units in vectorized code.

We would also like to enable AVX2, but not being wide spread enough it would require a new binary tag (to be agreed at LIM/AF).

See LBCORE-1227.

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