ARM: 9382/1: ftrace: Define ftrace_stub_graph

Several architectures defines this stub for the graph tracer,
and it is needed for CFI, as it needs a separate symbol for it.
The trick from include/asm-generic/vmlinux.lds.h to define
ftrace_stub_graph to ftrace_stub isn't working when using CFI.
Commit 883bbbff

 contains the details.

Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>