mk/spdk.common.mk: remove -std=c++0x

This isn't really needed, and actually hinders
things like Seastar which use more recent C++
variants.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I7ce3d0578d6832a5d2b26c661bf7a7e2119bcac5

Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/470079


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>