Fix gdbserver/linux target_waitstatus logging assert

Fix gdbserver/linux target_waitstatus logging assert

Turning on debug output in gdbserver leads to an assertion failure if
gdbserver reports a non-signal event:

    [threads] wait_1: LWP 3273770: extended event with waitstatus status->kind = EXECD, execd_pathname = gdb.threads/non-ldr-exc-1/non-ldr-exc-1
    [threads] wait_1: Hit a non-gdbserver trap event.
  ../../src/gdbserver/../gdb/target/waitstatus.h:365: A problem internal to GDBserver has been detected.
  sig: Assertion `m_kind == TARGET_WAITKIND_STOPPED || m_kind == TARGET_WAITKIND_SIGNALLED' failed.

Fix it in the obvious way, using target_waitstatus::to_string(),
resulting in, for example:

  [threads] wait_1: ret = LWP 1542412.1542412, status->kind = STOPPED, sig = GDB_SIGNAL_TRAP

Change-Id: Ia4832f9b4fa39f4af67fcaf21fd4d909a285a645