go.sys/plan9: define "use" in plan9/syscall.go

LGTM=0intro
R=rsc, 0intro
CC=golang-codereviews
https://golang.org/cl/141870043

plan9/syscall.go

@@ -21,6 +21,8 @@
 // holds a value of type syscall.ErrorString.
 package plan9
 
+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).
@@ -65,3 +67,8 @@ func (ts *Timespec) Nano() int64 {
 func (tv *Timeval) Nano() int64 {
 	return int64(tv.Sec)*1e9 + int64(tv.Usec)*1000
 }
+
+// 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)