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>

diff --git a/src/spdk b/src/spdk
index 7d45ab345d7293c6679bedd89d5dc16310026ba9..f60effdd75b782542a539eaad00d8f8f1d2df988 160000 (submodule)
--- a/src/spdk
+++ b/src/spdk
@@ -1 +1 @@
-Subproject commit 7d45ab345d7293c6679bedd89d5dc16310026ba9
+Subproject commit f60effdd75b782542a539eaad00d8f8f1d2df988