vmlinux.lds: fix .data..init_task output section (fix popwerpc boot)

The .data..init_task output section was missing
a load offset causing a popwerpc target to fail to boot.

Sean MacLennan tracked it down to the definition of
INIT_TASK_DATA_SECTION().

There are only two users of INIT_TASK_DATA_SECTION()
in the kernel today: cris and popwerpc.
cris do not support relocatable kernels and is thus not
impacted by this change.

Fix INIT_TASK_DATA_SECTION() to specify load offset like
all other output sections.

Reported-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

include/asm-generic/vmlinux.lds.h

@@ -435,7 +435,7 @@
  */
 #define INIT_TASK_DATA_SECTION(align)					\
 	. = ALIGN(align);						\
-	.data..init_task : {						\
+	.data..init_task :  AT(ADDR(.data..init_task) - LOAD_OFFSET) {	\
 		INIT_TASK_DATA(align)					\
 	}