arm64: ftrace: Define ftrace_stub_graph only with FUNCTION_GRAPH_TRACER

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/202210251659.tRMs78RH-lkp@intel.com/
Fixes: 883bbbffa5a4 ("ftrace,kcfi: Separate ftrace_stub() and ftrace_stub_graph()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20221109192831.3057131-1-samitolvanen@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>