[S390] Fix alignment of initial kernel stack.

We need an alignment of 16384 bytes for the initial kernel stack if
the kernel is configured for 16384 bytes stacks but the linker script
currently guarantees only an alignment of 8192 bytes.

So fix this and simply use THREAD_SIZE as alignment value which will
always do the right thing.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>