Micro-optimize timed wait loops. NFC (#26849)

Split out from #13007.

system/lib/pthread/library_pthread.c

diff --git a/system/lib/pthread/library_pthread.c b/system/lib/pthread/library_pthread.c
index a894258..0a47e1a 100644
--- a/system/lib/pthread/library_pthread.c
+++ b/system/lib/pthread/library_pthread.c
@@ -85,18 +85,17 @@
   __pthread_testcancel();
   if (msecs > 0) {
     uint32_t dummyZeroAddress = 0;
-    double start = emscripten_get_now();
-    double elapsed = 0;
-    while (elapsed < msecs) {
+    double target = emscripten_get_now() + msecs;
+    do {
       emscripten_conditional_set_current_thread_status(EM_THREAD_STATUS_RUNNING,
                                                        EM_THREAD_STATUS_SLEEPING);
-      emscripten_futex_wait(&dummyZeroAddress, 0, msecs - elapsed);
+      emscripten_futex_wait(&dummyZeroAddress, 0, msecs);
       emscripten_conditional_set_current_thread_status(EM_THREAD_STATUS_SLEEPING,
                                                        EM_THREAD_STATUS_RUNNING);
       emscripten_current_thread_process_queued_calls();
       __pthread_testcancel();
-      elapsed = emscripten_get_now() - start;
-    }
+      msecs = target - emscripten_get_now();
+    } while (msecs > 0);
   }
 }
 

system/lib/pthread/proxying_legacy.c

diff --git a/system/lib/pthread/proxying_legacy.c b/system/lib/pthread/proxying_legacy.c
index b9c0e14..f6dae33 100644
--- a/system/lib/pthread/proxying_legacy.c
+++ b/system/lib/pthread/proxying_legacy.c
@@ -530,24 +530,23 @@
 }
 
 EMSCRIPTEN_RESULT emscripten_wait_for_call_v(em_queued_call* call, double timeoutMSecs) {
-  int r;
-
-  int done = atomic_load(&call->operationDone);
-  if (!done) {
-    double now = emscripten_get_now();
-    double waitEndTime = now + timeoutMSecs;
-    emscripten_set_current_thread_status(EM_THREAD_STATUS_WAITPROXY);
-    while (!done && now < waitEndTime) {
-      r = emscripten_futex_wait(&call->operationDone, 0, waitEndTime - now);
-      done = atomic_load(&call->operationDone);
-      now = emscripten_get_now();
-    }
-    emscripten_set_current_thread_status(EM_THREAD_STATUS_RUNNING);
-  }
-  if (done)
+  if (call->operationDone) {
     return EMSCRIPTEN_RESULT_SUCCESS;
-  else
-    return EMSCRIPTEN_RESULT_TIMED_OUT;
+  }
+
+  emscripten_set_current_thread_status(EM_THREAD_STATUS_WAITPROXY);
+
+  int r;
+  double target = emscripten_get_now() + timeoutMSecs;
+  do {
+    r = -emscripten_futex_wait(&call->operationDone, 0, timeoutMSecs);
+
+    timeoutMSecs = target - emscripten_get_now();
+  } while (r == EINTR && timeoutMSecs > 0);
+
+  emscripten_set_current_thread_status(EM_THREAD_STATUS_RUNNING);
+
+  return r == ETIMEDOUT ? EMSCRIPTEN_RESULT_TIMED_OUT : EMSCRIPTEN_RESULT_SUCCESS;
 }
 
 EMSCRIPTEN_RESULT emscripten_wait_for_call_i(