Use VDSO for clock_gettime(2) and gettimeofday(2).

Bug: 15387103

(cherry picked from commit 625993dfbb085a3cde7492eda8ec1cdc1ee39a78)

Change-Id: I0e156d7049ba1495902259071a96936592e74025
diff --git a/libc/Android.mk b/libc/Android.mk
index 51002e5..0485d72 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -229,6 +229,7 @@
     bionic/umount.cpp \
     bionic/unlink.cpp \
     bionic/utimes.cpp \
+    bionic/vdso.cpp \
     bionic/vfork.cpp \
     bionic/wait.cpp \
     bionic/wchar.cpp \