gdb: use ptid_t::to_string in infrun debug messages

gdb: use ptid_t::to_string in infrun debug messages

In debug messages, I think it would be more helpful to print ptid using
the simple "pid.lwp.tid" notation in infrun debug messages.  I am
currently debugging some fork issues, and find the pid_to_str output not
so useful, as it doesn't tell which process a thread belongs to.

It currently shows up like this:

    [infrun] resume_1: step=1, signal=GDB_SIGNAL_0, trap_expected=0, current thread [Thread 0x7ffff7d95740 (LWP 892942)] at 0x55555555521f

With the patch, it shows up like this:

    [infrun] resume_1: step=1, signal=GDB_SIGNAL_0, trap_expected=1, current thread [894072.894077.0] at 0x5555555551d9

Change-Id: I130796d7dfb0d8e763b8358d8a6002701d80c4ea