Add support code for the canadian-cross case.

Add implementation for a candadian build option already
present in crosstool in order to build a cross-compiler
where build != host != target

Signed-off-by: Bart van der Meulen <bartvdrmeulen@gmail.com>

# Compute x86_64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}