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 M68k-specific values

CT_DoArchTupleValues() {

    :

}