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/libc/include/sys/epoll.h b/libc/include/sys/epoll.h
index 2bc16f5..a213a90 100644
--- a/libc/include/sys/epoll.h
+++ b/libc/include/sys/epoll.h
@@ -58,6 +58,7 @@
 int epoll_ctl(int __epoll_fd, int __op, int __fd, struct epoll_event* __event);
 int epoll_wait(int __epoll_fd, struct epoll_event* __events, int __event_count, int __timeout_ms);
 int epoll_pwait(int __epoll_fd, struct epoll_event* __events, int __event_count, int __timeout_ms, const sigset_t* __mask) __INTRODUCED_IN(21);
+int epoll_pwait64(int __epoll_fd, struct epoll_event* __events, int __event_count, int __timeout_ms, const sigset64_t* __mask) __INTRODUCED_IN(28);
 
 __END_DECLS