[hwasan] Flush stderr/stdout in tests (#114083)

The x86_64_lam_qemu buildbots started failing
(https://lab.llvm.org/buildbot/#/builders/139/builds/5462/steps/2/logs/stdio).
Based on the logs, it appears the HWASan check is correct but it did not
match the stderr/stdout output. This patch attempts to fix the issue by
flushing stderr/stdout as appropriate.

Author: thurstond

compiler-rt/test/hwasan/TestCases/many-threads-uaf.c

@@ -23,6 +23,7 @@ void *BoringThread(void *arg) {
 void *UAFThread(void *arg) {
   char * volatile x = (char*)malloc(10);
   fprintf(stderr, "ZZZ %p\n", x);
+  fflush(stderr);
   free(x);
   x[5] = 42;
   // CHECK: ERROR: HWAddressSanitizer: tag-mismatch on address

compiler-rt/test/hwasan/TestCases/mem-intrinsics.c

@@ -21,6 +21,7 @@ int main() {
   memcpy(Q, P, 32);
 #endif
   write(STDOUT_FILENO, "recovered\n", 10);
+  fflush(stdout);
   // WRITE: ERROR: HWAddressSanitizer: tag-mismatch on address
   // WRITE: WRITE of size 32 at {{.*}} tags: [[PTR_TAG:..]]/[[MEM_TAG:..]] (ptr/mem)
   // WRITE: Invalid access starting at offset 16

compiler-rt/test/hwasan/TestCases/use-after-free.c

@@ -15,6 +15,7 @@ int main() {
   free(x);
   __hwasan_disable_allocator_tagging();
   fprintf(stderr, ISREAD ? "Going to do a READ\n" : "Going to do a WRITE\n");
+  fflush(stderr);
   // CHECK: Going to do a [[TYPE:[A-Z]*]]
   int r = 0;
   if (ISREAD) r = x[5]; else x[5] = 42;  // should be on the same line.