arch: kill avr32

avr32 was slated for removal in 2015.02. Make it so!

This patch only definitively hides the symbol. When all references
to it are eradicated (to come in followup patches), we'll eventually
kill the symbol altogether.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>