samples: ftrace: Include the nospec-branch.h only for x86

When other architectures without the nospec functionality write their
direct-call functions of samples/ftrace/*.c, the including of
asm/nospec-branch.h must be taken care to fix the no header file found
error in building process.

This commit (ee3e246 "x86/ftrace: Make it call depth tracking aware")
file-globally includes asm/nospec-branch.h providing CALL_DEPTH_ACCOUNT
for only x86 direct-call functions.

It seems better to move the including to `#ifdef CONFIG_X86_64`.

Link: https://lore.kernel.org/linux-trace-kernel/20230130085954.647845-1-suagrfillet@gmail.com

Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Song Shuai

samples/ftrace/ftrace-direct-modify.c

@@ -3,7 +3,6 @@
 #include <linux/kthread.h>
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.h>
-#include <asm/nospec-branch.h>
 
 extern void my_direct_func1(void);
 extern void my_direct_func2(void);
@@ -26,6 +25,7 @@ static unsigned long my_ip = (unsigned long)schedule;
 #ifdef CONFIG_X86_64
 
 #include <asm/ibt.h>
+#include <asm/nospec-branch.h>
 
 asm (
 "	.pushsection    .text, \"ax\", @progbits\n"

samples/ftrace/ftrace-direct-multi-modify.c

@@ -3,7 +3,6 @@
 #include <linux/kthread.h>
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.h>
-#include <asm/nospec-branch.h>
 
 extern void my_direct_func1(unsigned long ip);
 extern void my_direct_func2(unsigned long ip);
@@ -24,6 +23,7 @@ extern void my_tramp2(void *);
 #ifdef CONFIG_X86_64
 
 #include <asm/ibt.h>
+#include <asm/nospec-branch.h>
 
 asm (
 "	.pushsection    .text, \"ax\", @progbits\n"

samples/ftrace/ftrace-direct-multi.c

@@ -5,7 +5,6 @@
 #include <linux/ftrace.h>
 #include <linux/sched/stat.h>
 #include <asm/asm-offsets.h>
-#include <asm/nospec-branch.h>
 
 extern void my_direct_func(unsigned long ip);
 
@@ -19,6 +18,7 @@ extern void my_tramp(void *);
 #ifdef CONFIG_X86_64
 
 #include <asm/ibt.h>
+#include <asm/nospec-branch.h>
 
 asm (
 "	.pushsection    .text, \"ax\", @progbits\n"

samples/ftrace/ftrace-direct-too.c

@@ -4,7 +4,6 @@
 #include <linux/mm.h> /* for handle_mm_fault() */
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.h>
-#include <asm/nospec-branch.h>
 
 extern void my_direct_func(struct vm_area_struct *vma,
 			   unsigned long address, unsigned int flags);
@@ -21,6 +20,7 @@ extern void my_tramp(void *);
 #ifdef CONFIG_X86_64
 
 #include <asm/ibt.h>
+#include <asm/nospec-branch.h>
 
 asm (
 "	.pushsection    .text, \"ax\", @progbits\n"

samples/ftrace/ftrace-direct.c

@@ -4,7 +4,6 @@
 #include <linux/sched.h> /* for wake_up_process() */
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.h>
-#include <asm/nospec-branch.h>
 
 extern void my_direct_func(struct task_struct *p);
 
@@ -18,6 +17,7 @@ extern void my_tramp(void *);
 #ifdef CONFIG_X86_64
 
 #include <asm/ibt.h>
+#include <asm/nospec-branch.h>
 
 asm (
 "	.pushsection    .text, \"ax\", @progbits\n"