gdb/darwin: skip over WIFSTOPPED wait4 status

gdb/darwin: skip over WIFSTOPPED wait4 status

On modern Darwin's, there appears to be a new circumstance in which a
MACH_NOTIFY_DEAD_NAME message can be received, and which was not
previously accounted for: to signal the WIFSTOPPED condition in the
debuggee. In that case the debuggee is not dead yet (and in fact,
counting it as dead would cause a zombie leak - A process in such a
state reparents to PID 1, but cannot be killed).

 - Read and ignore such messages (counting on the next exception message
   to let us know of the inferior's new state again)
 - Refactor logging so as to clearly distinguish between the
   MACH_NOTIFY_DEAD_NAME cases (WIFEXITED, WIFSTOPPED, signal, or
   something else), and warn in the last case

Co-authored-by: Louis-He <1726110778@qq.com>
Co-authored-by: Philippe Blain <levraiphilippeblain@gmail.com>
Change-Id: Ie86904a894e9bd154e6b674b1bfbfbaee7fde3e1