spdk: -march=core2 instead of -march=native

spdk: -march=core2 instead of -march=native

-march=native causes gcc to use opcodes according to whichever CPU happens to
be installed in the build host, which can be different for every build. This
makes it impossible to achieve a reproducible build.

Also, if the build host has a very new CPU, running the resulting binaries on
older CPUs (of the same family, i.e. x86_64) could result in segmentation
fault.

Hopefully nobody will be running Ceph on x86_64 CPUs older than Core2 (?)

Fixes: http://tracker.ceph.com/issues/24948
Signed-off-by: Nathan Cutler <ncutler@suse.com>