docs/known-issues: add the infamous "mixed implicit and normal rules"

make-3.82 now refuses to have both normal and implicit rules in the
same definition, so document the issue with a known workaround.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="dsreed on freenode/#crosstool-ng"

 reporter_url=""

 reporter_comment="Enables experimental support for VFP/NEON hardware floating point."