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.

Change-Id: Ib310ade3256712ca617a90539e8eb07459c98505
diff --git a/tests/Android.mk b/tests/Android.mk
index 491b13c..dee5e33 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -58,6 +58,7 @@
 
 test_src_files = \
     dirent_test.cpp \
+    eventfd_test.cpp \
     fenv_test.cpp \
     getauxval_test.cpp \
     getcwd_test.cpp \