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)

reporter_name="Ioannis E. VENETIS"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

reporter_comment=""