Move 64bit division from avr32 to generic lib

Move the 64bit division from lib_avr32 to lib_generic. With this, all
boards can do_div/__div64_32 if needed, not only avr one. Code is put
to lib_generic, so no larger memory footprint if not used. No code
modifications. Thanks for proposal by Håvard Skinnemoen.

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>