runtime: add comments for blank import of "unsafe"

Change-Id: Iaad548ea90d3b70341a723450d5e4d4c2b88856f
Reviewed-on: https://go-review.googlesource.com/2063Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/noasm.go

@@ -9,7 +9,7 @@
 
 package runtime
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 func cmpstring(s1, s2 string) int {
 	l := len(s1)

src/runtime/os_android_arm.go

@@ -4,7 +4,7 @@
 
 package runtime
 
-import _ "unsafe"
+import _ "unsafe" // for go:cgo_export_static and go:cgo_export_dynamic
 
 // Export the runtime entry point symbol.
 //

src/runtime/os_windows.go

@@ -4,7 +4,7 @@
 
 package runtime
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 type stdFunction *byte
 

src/runtime/syscall2_solaris.go

@@ -4,7 +4,7 @@
 
 package runtime
 
-import _ "unsafe"
+import _ "unsafe" // for go:linkname
 
 //go:cgo_import_dynamic libc_chdir chdir "libc.so"
 //go:cgo_import_dynamic libc_chroot chroot "libc.so"