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

Change-Id: Ide040884c456190226e580513099fdb8377e015b
diff --git a/libc/arch-x86/syscalls.mk b/libc/arch-x86/syscalls.mk
index 133f286..420a91e 100644
--- a/libc/arch-x86/syscalls.mk
+++ b/libc/arch-x86/syscalls.mk
@@ -175,3 +175,4 @@
 syscall_src += arch-x86/syscalls/inotify_add_watch.S
 syscall_src += arch-x86/syscalls/inotify_rm_watch.S
 syscall_src += arch-x86/syscalls/poll.S
+syscall_src += arch-x86/syscalls/eventfd.S