arch: add hidden config knobs for 32- and 64-bit support and select

Add new hidden config knobs that architectures can select to
say whether they support 32- or 64-bit variants, and which
they default to.

# Compute x86_64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}