ROHD-2434: -march implies -mtune, remove -mtune

This also solves the problem that specifying x96-64 as generic
optimisation works for -march=x86-64 but not for -mtune.


(cherry picked from commit 2ca76511)

Docker/lofar-base/Dockerfile.tmpl

@@ -71,7 +71,7 @@ ARG CXX_FLAGS="--std=c++11 -W -Wall -Woverloaded-virtual -Wno-unknown-pragmas -D
 # Allow to overwrite the CPU optimisation default setting by specifying
 # --build-arg CPU_OPTIMISATION="native"
 ARG CPU_OPTIMISATION="haswell"
-ENV CPU_OPTIMISATION="-march=${CPU_OPTIMISATION} -mtune=${CPU_OPTIMISATION}"
+ENV CPU_OPTIMISATION="-march=${CPU_OPTIMISATION}"
 
 # Combine CXX_FLAGS and CPU_OPTIMISATION
 ENV CXX_FLAGS="${CXX_FLAGS} ${CPU_OPTIMISATION}"