libtracecmd: Print call offset in parent function for funcgraph-retaddr

When the "funcgraph-retaddr" option is enabled, trace-cmd currently
prints only the parent function name using the return address.

Enhance it to include the call offset within the parent function,
enabling tools like faddr2line to locate the exact call site.

Before:
  preempt_count_add(val=65536) /* <-irq_enter_rcu */

After:
  preempt_count_add(val=65536) /* <-irq_enter_rcu+0x17 */

Example usage with faddr2line:
  $ ./scripts/faddr2line vmlinux irq_enter_rcu+0x17
  irq_enter_rcu+0x17/0x80:
  irq_enter_rcu at kernel/softirq.c:664

Link: https://lore.kernel.org/20260209043530.3860119-1-dolinux.peng@gmail.com
Signed-off-by: Donglin Peng <pengdonglin@xiaomi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>