scripts/functions: use portable 'gzip -dc'

'zcat' on MacOS-X is broken (it is not gzip's zcat, but compres' zcat).

Use 'gzip -dc' for portability, as suggested by Anthony.

Reported-by: Fernando Ortiz <fortiz2k@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Anthony Foiani <anthony.foiani@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${CT_ARCH_SUFFIX}"

     fi

 }