Construct init thread stack in the linker script rather than by union

Construct the init thread stack in the linker script rather than doing it
by means of a union so that ia64's init_task.c can be got rid of.

The following symbols are then made available from INIT_TASK_DATA() linker
script macro:

	init_thread_union
	init_stack

INIT_TASK_DATA() also expands the region to THREAD_SIZE to accommodate the
size of the init stack.  init_thread_union is given its own section so that
it can be placed into the stack space in the right order.  I'm assuming
that the ia64 ordering is correct and that the task_struct is first and the
thread_info second.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>
Tested-by: Will Deacon <will.deacon@arm.com> (arm64)
Tested-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>

arch/um/include/asm/processor-generic.h

@@ -58,7 +58,10 @@ static inline void release_thread(struct task_struct *task)
 {
 }
 
-#define init_stack	(init_thread_union.stack)
+static inline void mm_copy_segments(struct mm_struct *from_mm,
+				    struct mm_struct *new_mm)
+{
+}
 
 /*
  * User space process size: 3GB (default).

arch/um/include/asm/thread_info.h

@@ -6,6 +6,9 @@
 #ifndef __UM_THREAD_INFO_H
 #define __UM_THREAD_INFO_H
 
+#define THREAD_SIZE_ORDER CONFIG_KERNEL_STACK_ORDER
+#define THREAD_SIZE ((1 << CONFIG_KERNEL_STACK_ORDER) * PAGE_SIZE)
+
 #ifndef __ASSEMBLY__
 
 #include <asm/types.h>
@@ -37,10 +40,6 @@ struct thread_info {
 	.real_thread = NULL,			\
 }
 
-#define init_thread_info	(init_thread_union.thread_info)
-#define init_stack		(init_thread_union.stack)
-
-#define THREAD_SIZE ((1 << CONFIG_KERNEL_STACK_ORDER) * PAGE_SIZE)
 /* how to get the thread information struct from C */
 static inline struct thread_info *current_thread_info(void)
 {
@@ -53,8 +52,6 @@ static inline struct thread_info *current_thread_info(void)
 	return ti;
 }
 
-#define THREAD_SIZE_ORDER CONFIG_KERNEL_STACK_ORDER
-
 #endif
 
 #define TIF_SYSCALL_TRACE	0	/* syscall trace active */

arch/um/include/asm/vmlinux.lds.h

@@ -0,0 +1,2 @@
+#include <asm/thread_info.h>
+#include <asm-generic/vmlinux.lds.h>