DPDK is build with march=corei7

[Christian Ehrhardt ]

Initially we have taken care to only enable the default 64 bit cpu (march=x86_84 is the default) and SSE3 because it could not made to work without.
SSE3 was part of the MIR in bug 1492186.

It seems since then the build system sneaked in an optimization that we didn't intend.
See https://launchpadlibrarian.net/382624532/buildlog_ubuntu-bionic-amd64.dpdk_17.11.3-3~ubuntu0.18.04_BUILDING.txt.gz

There is -march=corei7 set by the DPDK build system.

This leads to a surely interesting discussion, we are now faced with:
- DPDK would not work (at least well) on processors not having the new CPU features
- Many people asked us to enable an optimization like that and we refused for "compatibility"
- it is that way since Bionic was released and we had no complaint at all

Therefore maybe the right thing is a doc fix that this has a higher CPU requirement than usual.
We do so for SSE3 already.

OTOH if we switch it off back to compatibility I'm sure some people will hit performance regressions.

Decision:
- switch back to Ubuntu default of x86_64 (+sse3)
- keep DPDK project default bas level (=corei7) and update documentation

I'll discuss that in the Team and the deb_dpdk project

[Christian Ehrhardt  (paelzer)]

Discussed with deb_dpdk co-maintainer and we both lean towards keeping it as is for
- not fighting upstream on this
- keep upstream baseline
- dpdk on super old machines makes no sense in general

Discussing again with a few more people, before changes (would take place on next stable releases).