x86/ftrace: Use Makefile logic instead of #ifdef for compiling ftrace_*.o

Currently ftrace_32.S and ftrace_64.S are compiled even when
CONFIG_FUNCTION_TRACER is not set. This means there's an unnecessary #ifdef
to protect the code. Instead of using preprocessor directives, only compile
those files when FUNCTION_TRACER is defined.

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20170316210043.peycxdxktwwn6cid@treble
Link: http://lkml.kernel.org/r/20170323143446.217684991@goodmis.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/Makefile

@@ -46,8 +46,7 @@ obj-$(CONFIG_MODIFY_LDT_SYSCALL)	+= ldt.o
 obj-y			+= setup.o x86_init.o i8259.o irqinit.o jump_label.o
 obj-$(CONFIG_IRQ_WORK)  += irq_work.o
 obj-y			+= probe_roms.o
-obj-$(CONFIG_X86_64)	+= sys_x86_64.o ftrace_64.o
-obj-$(CONFIG_X86_32)	+= ftrace_32.o
+obj-$(CONFIG_X86_64)	+= sys_x86_64.o
 obj-$(CONFIG_X86_ESPFIX64)	+= espfix_64.o
 obj-$(CONFIG_SYSFS)	+= ksysfs.o
 obj-y			+= bootflag.o e820.o
@@ -83,6 +82,7 @@ obj-y				+= apic/
 obj-$(CONFIG_X86_REBOOTFIXUPS)	+= reboot_fixups_32.o
 obj-$(CONFIG_DYNAMIC_FTRACE)	+= ftrace.o
 obj-$(CONFIG_LIVEPATCH)	+= livepatch.o
+obj-$(CONFIG_FUNCTION_TRACER)	+= ftrace_$(BITS).o
 obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += ftrace.o
 obj-$(CONFIG_FTRACE_SYSCALLS)	+= ftrace.o
 obj-$(CONFIG_X86_TSC)		+= trace_clock.o

arch/x86/kernel/ftrace_32.S

@@ -8,8 +8,6 @@
 #include <asm/export.h>
 #include <asm/ftrace.h>
 
-#ifdef CONFIG_FUNCTION_TRACER
-
 #ifdef CC_USING_FENTRY
 # define function_hook	__fentry__
 EXPORT_SYMBOL(__fentry__)
@@ -206,7 +204,6 @@ ftrace_stub:
 	jmp	ftrace_stub
 END(function_hook)
 #endif /* CONFIG_DYNAMIC_FTRACE */
-#endif /* CONFIG_FUNCTION_TRACER */
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 ENTRY(ftrace_graph_caller)

arch/x86/kernel/ftrace_64.S

@@ -11,9 +11,6 @@
 	.code64
 	.section .entry.text, "ax"
 
-
-#ifdef CONFIG_FUNCTION_TRACER
-
 #ifdef CC_USING_FENTRY
 # define function_hook	__fentry__
 EXPORT_SYMBOL(__fentry__)
@@ -295,7 +292,6 @@ trace:
 	jmp fgraph_trace
 END(function_hook)
 #endif /* CONFIG_DYNAMIC_FTRACE */
-#endif /* CONFIG_FUNCTION_TRACER */
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 ENTRY(ftrace_graph_caller)