Add `sigset64_t` and accompanying functions.

This doesn't address `struct sigaction` and `sigaction`. That will
come later.

Bug: http://b/72493232
Test: ran tests
Change-Id: I4134346757ce3a4dac6feae413361cec16223386
diff --git a/tests/sys_epoll_test.cpp b/tests/sys_epoll_test.cpp
index d1b411a..7233ccc 100644
--- a/tests/sys_epoll_test.cpp
+++ b/tests/sys_epoll_test.cpp
@@ -35,6 +35,11 @@
   // epoll_pwait without a sigset (which is equivalent to epoll_wait).
   ASSERT_EQ(0, epoll_pwait(epoll_fd, events, 1, 1, NULL));
 
+#if defined(__BIONIC__)
+  // epoll_pwait64 without a sigset (which is equivalent to epoll_wait).
+  ASSERT_EQ(0, epoll_pwait64(epoll_fd, events, 1, 1, NULL));
+#endif
+
   // epoll_pwait with a sigset.
   sigset_t ss;
   sigemptyset(&ss);