arm64: ftrace: Define ftrace_stub_graph only with FUNCTION_GRAPH_TRACER

The 0-day bot reports that arm64 builds with CONFIG_CFI_CLANG +
CONFIG_FTRACE are broken when CONFIG_FUNCTION_GRAPH_TRACER is not
enabled:

 ld.lld: error: undefined symbol: __kcfi_typeid_ftrace_stub_graph
 >>> referenced by entry-ftrace.S:299 (arch/arm64/kernel/entry-ftrace.S:299)
 >>>               arch/arm64/kernel/entry-ftrace.o:(.text+0x48) in archive vmlinux.a

This is caused by ftrace_stub_graph using SYM_TYPE_FUNC_START when
the address of the function is not taken in any C translation unit.

Fix the build by only defining ftrace_stub_graph when it's actually
needed, i.e. with CONFIG_FUNCTION_GRAPH_TRACER.

Link: https://lore.kernel.org/lkml/[email protected]/
Fixes: 883bbbf ("ftrace,kcfi: Separate ftrace_stub() and ftrace_stub_graph()")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Sami Tolvanen <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Reviewed-by: Masami Hiramatsu (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: samitolvanen

arch/arm64/kernel/entry-ftrace.S

@@ -299,11 +299,11 @@ SYM_TYPED_FUNC_START(ftrace_stub)
 	ret
 SYM_FUNC_END(ftrace_stub)
 
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
 SYM_TYPED_FUNC_START(ftrace_stub_graph)
 	ret
 SYM_FUNC_END(ftrace_stub_graph)
 
-#ifdef CONFIG_FUNCTION_GRAPH_TRACER
 /*
  * void return_to_handler(void)
  *