avr32: Fix .size directive for cpu_enter_idle

gas used to accept (and ignore?) .size directives which referred to
undefined symbols, as this does.  In binutils 2.21 these are treated
as errors.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>