libc/eglibc: Make eglibc 2.11 and 2.12 not experimental

I haven't noticed the usual experimental testers complain about eglibc
2.11 or 2.12 being unstable. So stop marking them as experimental.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }