unix: add Eventfd function on Linux

Add support for the eventfd2 syscall on Linux.

Use the eventfd2 syscall instead of eventfd, as the latter does not
provide a flags argument and glibc also maps its eventfd() function to
the eventfd2 syscall, see the Notes section in
http://man7.org/linux/man-pages/man2/eventfd.2.html

Also add the corresponding flags values EFD_CLOEXEC, EFD_NONBLOCK and
EFD_SEMAPHORE.

Change-Id: Ia8c99e68d5966ab5c7ebe7e45423943fd7d8214e
Reviewed-on: https://go-review.googlesource.com/45093
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -1171,6 +1171,7 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sysnb	EpollCreate(size int) (fd int, err error)
 //sysnb	EpollCreate1(flag int) (fd int, err error)
 //sysnb	EpollCtl(epfd int, op int, fd int, event *EpollEvent) (err error)
+//sys	Eventfd(initval uint, flags int) (fd int, err error) = SYS_EVENTFD2
 //sys	Exit(code int) = SYS_EXIT_GROUP
 //sys	Faccessat(dirfd int, path string, mode uint32, flags int) (err error)
 //sys	Fallocate(fd int, mode uint32, off int64, len int64) (err error)
@@ -1316,7 +1317,6 @@ func Vmsplice(fd int, iovs []Iovec, flags int) (int, error) {
 // EpollCtlOld
 // EpollPwait
 // EpollWaitOld
-// Eventfd
 // Execve
 // Fgetxattr
 // Flistxattr