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 Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_SUFFIX:-${CT_ARCH_ALPHA_VARIANT}}"

}