Prevent name conflict for eventfd.cpp and eventfd.s when building libc.a

- eventfd.cpp and eventfd.s will output to the same file when building libc.a
   out/target/product/*/obj/STATIC_LIBRARIES/libc_intermediates/WHOLE/libc_common_objs/eventfd.o
 - And then `eventfd` will undefined when statically linked to libc.

Also add a unit test.

(cherry-pick of 8baa929d5d3bcf63381cf78ba76168c80c303f5e.)

Change-Id: Icd0eb0f4ce0511fb9ec00a504d491afd47d744d3

tests/eventfd_test.cpp

@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2013 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <gtest/gtest.h>
+
+#if !defined(__GLIBC__) // Android's prebuilt gcc's header files don't include <sys/eventfd.h>.
+
+#include <sys/eventfd.h>
+
+TEST(eventfd, smoke) {
+  unsigned int initial_value = 2;
+  int fd = eventfd(initial_value, O_NONBLOCK);
+  ASSERT_NE(fd, -1);
+
+  eventfd_t value = 123;
+  ASSERT_EQ(0, eventfd_read(fd, &value));
+  ASSERT_EQ(initial_value, value);
+
+  // Reading clears the counter.
+  ASSERT_EQ(-1, eventfd_read(fd, &value));
+  ASSERT_EQ(EAGAIN, errno);
+
+  // Values written are added until the next read.
+  ASSERT_EQ(0, eventfd_write(fd, 1));
+  ASSERT_EQ(0, eventfd_write(fd, 1));
+  ASSERT_EQ(0, eventfd_write(fd, 1));
+
+  ASSERT_EQ(0, eventfd_read(fd, &value));
+  ASSERT_EQ(3U, value);
+
+  close(fd);
+}
+
+#endif