sparc: use .data.init_task section for init_thread_union

Use a dedicated aligned section for the init_thread_union
variable and declare this section in vmlinux.lds.

This align sparc with most other architectures.  Eventually this allow
the init_task bits to be unified across all architectures.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>