[compiler-rt] Reduce the number of threads in gcov test to avoid failure

[calixte]

Summary

Patch in D78477 introduced a new test for gcov and this test is failing on arm:

• http://lab.llvm.org:8011/builders/clang-cmake-thumbv7-full-sh/builds/4752/steps/ninja%20check%202/logs/stdio
  • http://lab.llvm.org:8011/builders/clang-cmake-armv7-full/builds/10501/steps/ninja%20check%202/logs/stdio

So try to fix it in reducing the number of threads.

compiler-rt/test/profile/Inputs/instrprof-gcov-multithread_fork.cpp

 #include <sys/types.h>
 #include <thread>
 #include <unistd.h>
-#include <vector>
 
 template <typename T>
 void launcher(T func) {
-  std::vector<std::thread> pool;
+  auto t1 = std::thread(func);
+  auto t2 = std::thread(func);
 
-  for (int i = 0; i < 10; i++) {
-    pool.emplace_back(std::thread(func));
-  }
+  t1.join();
+  t2.join();
 
-  for (auto &t : pool) {
-    t.join();
-  }
-}
-
-void h() {}
-
-void g() {
-  fork();
-  launcher<>(h);
-}
+void g() {}
 
 void f() {
   fork();
   launcher<>(g);
 }
 
 int main() {
   launcher<>(f);
 
   return 0;
 }