powerpc/64: move interrupt return asm to interrupt_64.S

The next patch would like to move interrupt return assembly code to a low
location before general text, so move it into its own file and include via
head_64.S
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-7-npiggin@gmail.com

arch/powerpc/include/asm/head-64.h

@@ -16,7 +16,7 @@
 	.section ".head.data.\name\()","a",@progbits
 .endm
 .macro use_ftsec name
-	.section ".head.text.\name\()"
+	.section ".head.text.\name\()","ax",@progbits
 .endm
 
 /*

arch/powerpc/kernel/head_64.S

@@ -194,8 +194,9 @@ CLOSE_FIXED_SECTION(first_256B)
 
 /* This value is used to mark exception frames on the stack. */
 	.section ".toc","aw"
+/* This value is used to mark exception frames on the stack. */
 exception_marker:
-	.tc	ID_72656773_68657265[TC],0x7265677368657265
+	.tc	ID_EXC_MARKER[TC],STACK_FRAME_REGS_MARKER
 	.previous
 
 /*
@@ -211,6 +212,8 @@ OPEN_TEXT_SECTION(0x100)
 
 USE_TEXT_SECTION()
 
+#include "interrupt_64.S"
+
 #ifdef CONFIG_PPC_BOOK3E
 /*
  * The booting_thread_hwid holds the thread id we want to boot in cpu