Make zap() more reliable

Make zap() more reliable

The gcc assembly version of zap() could still be optimized out under
gcc 5.1 or later, and the krb5int_zap() function could be optimized
out with link-time optimization.  Based on work by Zhaomo Yang and
Brian Johannesmeyer, use the C11 memset_s() when available, then fall
back to a memory barrier with gcc or clang, and finally fall back to
using krb5int_zap().  Modify krb5int_zap() to use a volatile pointer
in case link-time optimization is used.

(cherry picked from commit c163275f899b201dc2807b3ff2949d5e2ee7d838)

ticket: 8514
version_fixed: 1.15