powerpc: Redefine TIF_32BITS thread flag

Moving TIF_32BIT to use bit 20 instead of 4 in the task flag field.

This change is making room for an upcoming new task macro
(_TIF_SYSCALL_EMU) which is preferred to set a bit in the lower 16-bits
part of the word.

This upcoming flag macro will take part in a composed macro
(_TIF_SYSCALL_DOTRACE) which will contain other flags as well, and it is
preferred that the whole _TIF_SYSCALL_DOTRACE macro only sets the lower 16
bits of a word, so, it could be handled using immediate operations (as load
immediate, add immediate, ...) where the immediate operand (SI) is limited
to 16-bits.

Another possible solution would be using the LOAD_REG_IMMEDIATE() macro
to load a full 64-bits word immediate, but it takes 5 operations instead of
one.

Having TIF_32BITS being redefined to use an upper bit is not a problem
since there is only one place in the assembly code where TIF_32BIT is being
used, and it could be replaced with an operation with right shift (addis),
since it is used alone, i.e. not being part of a composed macro, which has
different bits set, and would require LOAD_REG_IMMEDIATE().

Tested on a 64 bits Big Endian machine running a 32 bits task.
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/thread_info.h

@@ -81,7 +81,6 @@ extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src
 #define TIF_SIGPENDING		1	/* signal pending */
 #define TIF_NEED_RESCHED	2	/* rescheduling necessary */
 #define TIF_FSCHECK		3	/* Check FS is USER_DS on return */
-#define TIF_32BIT		4	/* 32 bit binary */
 #define TIF_RESTORE_TM		5	/* need to restore TM FP/VEC/VSX */
 #define TIF_PATCH_PENDING	6	/* pending live patching update */
 #define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
@@ -100,6 +99,7 @@ extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src
 #define TIF_ELF2ABI		18	/* function descriptors must die! */
 #endif
 #define TIF_POLLING_NRFLAG	19	/* true if poll_idle() is polling TIF_NEED_RESCHED */
+#define TIF_32BIT		20	/* 32 bit binary */
 
 /* as above, but as bit values */
 #define _TIF_SYSCALL_TRACE	(1<<TIF_SYSCALL_TRACE)

arch/powerpc/kernel/entry_64.S

@@ -171,7 +171,7 @@ system_call:			/* label this so stack traces look sane */
  * based on caller's run-mode / personality.
  */
 	ld	r11,SYS_CALL_TABLE@toc(2)
-	andi.	r10,r10,_TIF_32BIT
+	andis.	r10,r10,_TIF_32BIT@h
 	beq	15f
 	addi	r11,r11,8	/* use 32-bit syscall entries */
 	clrldi	r3,r3,32