powerpc/64: Allow for THREAD_SIZE > 16k

Fix an assembler error when the THREAD_SIZE is greater than 16k.

Signed-off-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>