cmake : avoid -march=native when reproducible build is wanted (llama/11366)

cmake : avoid -march=native when reproducible build is wanted (llama/11366)

See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Without this patch, compiling on different machines produced different binaries, which made verification of results difficult.

Fixes: #11317
This patch was done while working on reproducible builds for openSUSE.