ProcessStub: limit ptrace only to Linux

The current code tries to use Linux ptrace on any OSes different than
Windows and macOS, leading to build failure on other non-Linux OSes.

Fix this in a couple of places:
- in onInferiorStarted(), create a new Linux block for the code not run
  on Windows and macOS
- in onInferiorStarted(), add a new block for any other OS to do the
  needed sendPid() call
- in setupUnixInferior(), drop the global !Q_OS_WIN, and limit the
  setChildProcessModifier() call with ptrace calls to Linux

Change-Id: Idfde3ee890eb94c6972343b70d0fe639a36343ae
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/tools/process_stub/main.cpp

@@ -254,8 +254,7 @@ void onInferiorStarted()
     // In debug mode we use the poll timer to send the pid.
     if (!debugMode)
         sendPid(inferiorId);
-#else
-
+#elif defined(Q_OS_LINUX)
     if (debugMode) {
         qCInfo(log) << "Waiting for SIGTRAP from inferiors execve ...";
         if (!waitFor(SIGTRAP))
@@ -271,12 +270,13 @@ void onInferiorStarted()
 
     qCInfo(log) << "Sending pid:" << inferiorId;
     sendPid(inferiorId);
+#else
+    sendPid(inferiorId);
 #endif
 }
 
 void setupUnixInferior()
 {
-#ifndef Q_OS_WIN
     if (debugMode) {
         qCInfo(log) << "Debug mode enabled";
 #ifdef Q_OS_DARWIN
@@ -287,7 +287,7 @@ void setupUnixInferior()
             // Suspend ourselves ...
             raise(SIGSTOP);
         });
-#else
+#elif defined(Q_OS_LINUX)
         // PTRACE_TRACEME will stop execution of the child process as soon as execve is called.
         inferiorProcess.setChildProcessModifier([] {
             ptrace(PTRACE_TRACEME, 0, 0, 0);
@@ -296,7 +296,6 @@ void setupUnixInferior()
         });
 #endif
     }
-#endif
 }
 
 void setupWindowsInferior()