buildbot: don't add ports that declare not to support ppc to the build queue of 10.5-ppc

[mojca (Mojca Miklavec)]

The 10.6-i386 buildbot avoids building many packages:

if [[ $exclude -eq 0 && "${os_version}" = "10.6" && "${is_64bit_capable}" = "0" ]]; then
    supported_archs=$("${option_prefix}/bin/port-tclsh" "${thisdir}/tools/supported-archs.tcl" "${port}")
    if [[ -n "${supported_archs}" && ! ("${supported_archs}" == *"x86_64"* && "${supported_archs}" == *"i386"*) ]]; then
        exclude=1
        exclude_reasons+=("the ${os_version}_x86_64 builder will build it")
    fi
fi

I believe that a similar strategy could be used to avoid building packages that are declared not to be supported on PPC.

See ​an example of the libvpx port which doesn't even start to build:

Error: libvpx cannot be installed for the configured build_arch 'ppc' because it only supports the arch(s) 'x86_64 i386'.
Error: Processing of port libvpx failed
Build of 'libvpx' failed.