libc: add <sys/eventfd.h> and corresponding implementations.

Change-Id: Ide040884c456190226e580513099fdb8377e015b
diff --git a/libc/arch-sh/syscalls.mk b/libc/arch-sh/syscalls.mk
index 47b5a1e..a87419d 100644
--- a/libc/arch-sh/syscalls.mk
+++ b/libc/arch-sh/syscalls.mk
@@ -161,3 +161,4 @@
 syscall_src += arch-sh/syscalls/inotify_add_watch.S
 syscall_src += arch-sh/syscalls/inotify_rm_watch.S
 syscall_src += arch-sh/syscalls/poll.S
+syscall_src += arch-sh/syscalls/eventfd.S