riscv: Add ftrace_stub_graph

Commit 883bbbf ("ftrace,kcfi: Separate ftrace_stub() and
ftrace_stub_graph()") added a separate ftrace_stub_graph function for
CFI_CLANG. Add the stub to fix FUNCTION_GRAPH_TRACER compatibility
with CFI.

Reviewed-by: Kees Cook <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Sami Tolvanen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: samitolvanen

arch/riscv/kernel/mcount.S

@@ -57,6 +57,10 @@ SYM_TYPED_FUNC_START(ftrace_stub)
 SYM_FUNC_END(ftrace_stub)
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
+SYM_TYPED_FUNC_START(ftrace_stub_graph)
+	ret
+SYM_FUNC_END(ftrace_stub_graph)
+
 ENTRY(return_to_handler)
 /*
  * On implementing the frame point test, the ideal way is to compare the