powerpc/32: Add EXCEPTION_PROLOG_0 in head_32.h

This patch creates a macro for the very first part of
exception prolog, this will help when implementing
CONFIG_VMAP_STACK
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2249fe62f481121a180e9655ad2b998093f318f3.1576916812.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/head_32.S

@@ -272,9 +272,7 @@ __secondary_hold_acknowledge:
  */
 	. = 0x200
 	DO_KVM  0x200
-	mtspr	SPRN_SPRG_SCRATCH0,r10
-	mtspr	SPRN_SPRG_SCRATCH1,r11
-	mfcr	r10
+	EXCEPTION_PROLOG_0
 #ifdef CONFIG_PPC_CHRP
 	mfspr	r11, SPRN_SPRG_THREAD
 	lwz	r11, RTAS_SP(r11)

arch/powerpc/kernel/head_32.h

@@ -10,13 +10,16 @@
  * We assume sprg3 has the physical address of the current
  * task's thread_struct.
  */
-
 .macro EXCEPTION_PROLOG
+	EXCEPTION_PROLOG_0
+	EXCEPTION_PROLOG_1
+	EXCEPTION_PROLOG_2
+.endm
+
+.macro EXCEPTION_PROLOG_0
 	mtspr	SPRN_SPRG_SCRATCH0,r10
 	mtspr	SPRN_SPRG_SCRATCH1,r11
 	mfcr	r10
-	EXCEPTION_PROLOG_1
-	EXCEPTION_PROLOG_2
 .endm
 
 .macro EXCEPTION_PROLOG_1

arch/powerpc/kernel/head_8xx.S

@@ -494,10 +494,7 @@ InstructionTLBError:
  */
 	. = 0x1400
 DataTLBError:
-	mtspr	SPRN_SPRG_SCRATCH0, r10
-	mtspr	SPRN_SPRG_SCRATCH1, r11
-	mfcr	r10
-
+	EXCEPTION_PROLOG_0
 	mfspr	r11, SPRN_DAR
 	cmpwi	cr0, r11, RPN_PATTERN
 	beq-	FixupDAR	/* must be a buggy dcbX, icbi insn. */
@@ -530,9 +527,7 @@ DARFixed:/* Return from dcbx instruction bug workaround */
  */
 	. = 0x1c00
 DataBreakpoint:
-	mtspr	SPRN_SPRG_SCRATCH0, r10
-	mtspr	SPRN_SPRG_SCRATCH1, r11
-	mfcr	r10
+	EXCEPTION_PROLOG_0
 	mfspr	r11, SPRN_SRR0
 	cmplwi	cr0, r11, (.Ldtlbie - PAGE_OFFSET)@l
 	cmplwi	cr7, r11, (.Litlbie - PAGE_OFFSET)@l