config: enable STACKPROTECTOR_STRONG (jsc#SLE-9120 bsc#1130365)
    
    Enable CONFIG_STACKPROTECTOR_STRONG on all architectures except s390x
    (where the feature is not available). This extends the number of functions
    which are protected by "stack canary" check to catch functions writing past
    their stack frame.
    
    This change was requested by SUSE security to make our kernels more
    resistant to some types of stack overflow attacks. Tests performed by
    kernel performance teams confirmed that performance impact is acceptable.