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.go

@@ -21,8 +21,6 @@
 // holds a value of type syscall.Errno.
 package unix // import "golang.org/x/sys/unix"
 
-import "unsafe"
-
 // ByteSliceFromString returns a NUL-terminated slice of bytes
 // containing the text of s. If s contains a NUL byte at any
 // location, it returns (nil, EINVAL).
@@ -69,8 +67,3 @@ func (tv *Timeval) Nano() int64 {
 }
 
 func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }
-
-// use is a no-op, but the compiler cannot see that it is.
-// Calling use(p) ensures that p is kept live until that point.
-//go:noescape
-func use(p unsafe.Pointer)