ptrace: Stop a traced thread when it exists from execve

This was a missing feature in the PT_TRACEME command.

This feature allows the tracer to interact with the tracee before the
tracee has started executing its program.

It will be useful for automatically inserting a breakpoint at a
debugged program's entry point.

Demos/Debugee/main.cpp

@@ -1,8 +1,12 @@
 #include <cstdio>
 int main(int, char**)
 {
-    printf("before breakpoint\n");
-    asm("int3");
-    printf("after breakpoint\n");
+    printf("Debuggee main\n");
+    int s = 0;
+    for (int i = 0; i < 10; ++i) {
+        s++;
+    }
+    printf("s: %d\n", s);
+    // asm("int3");
     return 0;
 }