tracing/function-graph: x86_64 stack allocation cleanup

Only 24 bytes needs to be reserved on the stack for the function graph
tracer on x86_64.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
LKML-Reference: <20090729085837.GB4998@jolsa.lab.eng.brq.redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>