[compiler-rt][Profile] Wait for child threads in set-file-object test

[DavidSpickett]

Summary

We've been seeing this test return 31 instead of 32 for the "functions"
line in this test on our AArch64 bots.

One possible cause is some of the children not finishing in time
before the llvm-profdata commands are run, if the machine is heavily loaded.

Wait for all the children to finish before exiting the parent.

[DavidSpickett]

Rebase onto main.

[DavidSpickett]

Temporarily disabled the test for arm linux in 8b86f8a3256a59cbaa12858cb0842025d48f549f.

[zequanwu]

LGTM, Thanks!

[zequanwu]

I committed on your behalf to unblock the bots.

compiler-rt/test/profile/ContinuousSyncMode/set-file-object.c

 // ZERO:     Function count: 0
 // ZERO:     Block counts: []
 // ZERO: Instrumentation level: Front-end
 
 #include <spawn.h>
 #include <stdio.h>
 #include <string.h>
 
+#include <sys/types.h>
+#include <sys/wait.h>
+
 const int num_child_procs_to_spawn = 32;
 
 extern int __llvm_profile_is_continuous_mode_enabled(void);
 extern int __llvm_profile_set_file_object(FILE *, int);
 
 int coverage_test() {
   return 0;
 }
for (I = 0; I < num_child_procs_to_spawn; ++I) {
       int ret =
           posix_spawn(&child_pids[I], argv[0], NULL, NULL, child_argv, child_envp);
       if (ret != 0) {
         fprintf(stderr, "Child %d could not be spawned: ret = %d, msg = %s\n",
                 I, ret, strerror(ret));
         return 1;
       }
     }
+    for (I = 0; I < num_child_procs_to_spawn; ++I) {
+      int status;
+      pid_t waited_pid = waitpid(child_pids[I], &status, 0);
+      if (waited_pid != child_pids[I]) {
+        fprintf(stderr, "Failed to wait on child %d\n", I);
+        return 1;
+      }
+      if (!WIFEXITED(status)) {
+        fprintf(stderr, "Child %d did not terminate normally\n", I);
+        return 1;
+      }
+      int return_status = WEXITSTATUS(status);
+      if (return_status != 0) {
+        fprintf(stderr, "Child %d exited with non zero status %d\n", I,
+                return_status);
+        return 1;
+      }
+    }
   } else if (strcmp(argv[1], "set") == 0) {
     // Child processes.
     if (!__llvm_profile_is_continuous_mode_enabled()) {
       fprintf(stderr, "Continuous mode disabled\n");
       return 1;
     }
     if (__llvm_profile_set_file_object(file, 1)) {
       fprintf(stderr, "Call to __llvm_profile_set_file_object failed\n");
       return 1;
     }
     // After set file object, counter should be written into new file.
     coverage_test();
   }
   return 0;
 }