Add renameat2.

Bug: http://b/127675384
Test: new tests
Change-Id: Ia2e3d5679180391ca98e62fa429fa11cbf167507
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 99bdc93..62d698f 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -151,6 +151,7 @@
 int mknodat(int, const char*, mode_t, dev_t)  all
 int readlinkat(int, const char*, char*, size_t)  all
 int renameat(int, const char*, int, const char*)  all
+int renameat2(int, const char*, int, const char*, unsigned)  all
 int symlinkat(const char*, int, const char*)  all
 int unlinkat(int, const char*, int)   all
 int utimensat(int, const char*, const struct timespec times[2], int)  all