arch: remove ARCH_TASK_STRUCT_ON_STACK

IA-64 was the only architecture which selected ARCH_TASK_STRUCT_ON_STACK.
IA-64 was removed with commit cf8e8658 ("arch: Remove Itanium (IA-64)
architecture"). Therefore remove support for ARCH_TASK_STRUCT_ON_STACK
as well.

Note: this also reveals a potential bug in powerpc code, which makes use of
__init_task_data without selecting ARCH_TASK_STRUCT_ON_STACK which makes
__init_task_data a no-op. This is broken since commit d11ed3ab ("Expand
INIT_TASK() in init/init_task.c and remove") from 2018 and needs to be
addressed separately.

Link: https://lkml.kernel.org/r/20231116133638.1636277-4-hca@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>