If the 'old' operand to cmpxchg() is a constant wider than 21 bits,
linking fails with a "relocation truncated to fit: R_AVR32_21S" error.
Fix this by replacing the "i" constraint with "Ks21" which makes the
compiler use a temporary register for out of range constants.
Signed-off-by: Mans Rullgard <mans@xxxxxxxxx>