complibs/isl: add HTTP mirror

Add well-known HTTP mirror as a fallback. This lets crosstool-ng
work when behind a HTTP/HTTPS only proxy.

Signed-off-by: Michael Hope <michaelh@juju.net.nz>
[me: split original patch in two]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <aeb4a850d0786ee62dc2.1375559989@wanda>
Patchwork-Id: 264436

 # 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

 }