[dev.cc] runtime/cgo: add comment about import _ "unsafe"

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/167650043

src/runtime/cgo/dragonfly.go

@@ -6,7 +6,7 @@
 
 package cgo
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 // Supply environ and __progname, because we don't
 // link against the standard DragonFly crt0.o and the

src/runtime/cgo/freebsd.go

@@ -6,7 +6,7 @@
 
 package cgo
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 // Supply environ and __progname, because we don't
 // link against the standard FreeBSD crt0.o and the

src/runtime/cgo/iscgo.go

@@ -11,7 +11,7 @@
 
 package cgo
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 //go:linkname _iscgo runtime.iscgo
 var _iscgo bool = true

src/runtime/cgo/netbsd.go

@@ -6,7 +6,7 @@
 
 package cgo
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 // Supply environ and __progname, because we don't
 // link against the standard NetBSD crt0.o and the

src/runtime/cgo/openbsd.go

@@ -6,7 +6,7 @@
 
 package cgo
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 // Supply environ, __progname and __guard_local, because
 // we don't link against the standard OpenBSD crt0.o and

src/runtime/cgo/setenv.go

@@ -6,7 +6,7 @@
 
 package cgo
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 //go:cgo_import_static x_cgo_setenv
 //go:linkname x_cgo_setenv x_cgo_setenv