[stage 3 subarch for intel 7th gen]

Hi,

Both will work. If you leave haswell you'll be fine.

According to the below: 

https://en.wikichip.org/wiki/intel/microarchitectures/kaby_lake

it should be: -march=skylake

I always check the output with the compiler (you can check booting livecd sysrecuecd)

gcc -march=native -E -v - </dev/null 2>&1 | grep cc1

or if you wish:[ CFLAGS ]

put this in make.conf

CFLAGS="-O2 -pipe -march=native" 
CXXFLAGS="${CFLAGS}"

and rebuild world. It shouldn't take long. ;-)