x/sys/unix: remove the "use" function and its calls.

Update mksyscall*.pl to skip generating calls to "use".
Make build tag handling match what's used in the syscall package.

See issue https://golang.org/issue/16607
and CL https://golang.org/cl/36616

Change-Id: I2d2e823fe1846d2110c07eb1bf976852706ecf60
Reviewed-on: https://go-review.googlesource.com/36715
Run-TryBot: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_dragonfly.go

@@ -109,7 +109,6 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 		bufsize = unsafe.Sizeof(Statfs_t{}) * uintptr(len(buf))
 	}
 	r0, _, e1 := Syscall(SYS_GETFSSTAT, uintptr(_p0), bufsize, uintptr(flags))
-	use(unsafe.Pointer(_p0))
 	n = int(r0)
 	if e1 != 0 {
 		err = e1