docs/known-issues: add note about build issue on x86_64 with 32bit userspace

Signed-off-by: Johannes Stezenbach <js@sig21.net>
(transplanted from da77b5e7cb9f921d0e2a22818b7945906100e704)

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

}