mk: rework cpu flags detection

mk: rework cpu flags detection

For cases where the compilation microarchitecture is explicitly given, we
extract the cpu-flags to use from the compiler rather than hard-coding. This
means that we will only ever use instruction sets supported by the compiler,
rather than having a case where the uarch and the Intel DPDK both support a
given instruction-set, but the compiler does not.
In the case where 'native' uarch support is requested, the same mechanism is
also used to detect the instruction-sets supported

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: David Marchand <david.marchand@6wind.com>