Add new __init_task_data macro to be used in arch init_task.c files.

Add new __init_task_data macro to be used in arch init_task.c files.

This patch is preparation for replacing most ".data.init_task" in the
kernel with macros, so that the section name can later be changed
without having to touch a lot of the kernel.

The long-term goal here is to be able to change the kernel's magic
section names to those that are compatible with -ffunction-sections
-fdata-sections.  This requires renaming all magic sections with names
of the form ".data.foo".

Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>