ftrace, ia64: Add macro for ftrace_caller

Define FTRACE_ADDR. In IA64, a function pointer isn't a 'unsigned long' but a
'struct {unsigned long ip, unsigned long gp}'.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>