unix: add Statx on Linux

statx(2) allows to get enhanced file status information (not currently
available through the existing stat syscalls), lightweight stat,
heavyweight stat. The Statx_t type used by Statx has consistent field
sizes on all arches (with year-2038-capable timestamps) and is closer to
the BSD implementation of Stat_t.

See http://man7.org/linux/man-pages/man2/statx.2.html for details. The
syscall was added in Linux kernel 4.11.

See https://github.com/tklauser/statx for an example of how this
function and types can be used to report stat(1)-like file status
information.

Change-Id: I9e9642b5b42a393f94fd532453888ce9ba4f0003
Reviewed-on: https://go-review.googlesource.com/87555
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -1318,6 +1318,7 @@ func Setgid(uid int) (err error) {
 
 //sys	Setpriority(which int, who int, prio int) (err error)
 //sys	Setxattr(path string, attr string, data []byte, flags int) (err error)
+//sys	Statx(dirfd int, path string, flags int, mask int, stat *Statx_t) (err error)
 //sys	Sync()
 //sys	Syncfs(fd int) (err error)
 //sysnb	Sysinfo(info *Sysinfo_t) (err error)