fgraph: Fix function type mismatches of ftrace_graph_return using ftrace_stub

The C compiler is allowing more checks to make sure that function pointers
are assigned to the correct prototype function. Unfortunately, the function
graph tracer uses a special name with its assigned ftrace_graph_return
function pointer that maps to a stub function used by the function tracer
(ftrace_stub). The ftrace_graph_return variable is compared to the
ftrace_stub in some archs to know if the function graph tracer is enabled or
not. This means we can not just simply create a new function stub that
compares it without modifying all the archs.

Instead, have the linker script create a function_graph_stub that maps to
ftrace_stub, and this way we can define the prototype for it to match the
prototype of ftrace_graph_return, and make the compiler checks all happy!

Link: http://lkml.kernel.org/r/20191015090055.789a0aed@gandalf.local.home

Cc: linux-sh@vger.kernel.org
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc:  Rich Felker <dalias@libc.org>
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>