runtime: say where the compiler knows about var writeBarrier

The runtime.writeBarrier variable tries to be helpful by telling you
that the compiler also knows about this variable, which you could
probably guess, but doesn't say how the compiler knows about it. In
fact, the compiler has a complete copy in builtin/runtime.go that
needs to be kept in sync. Say so.

Change-Id: Ia7fb0c591cb6f9b8230decce01008b417dfcec89
Reviewed-on: https://go-review.googlesource.com/40150
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgc.go

@@ -238,7 +238,9 @@ func setGCPercent(in int32) (out int32) {
 var gcphase uint32
 
 // The compiler knows about this variable.
-// If you change it, you must change the compiler too.
+// If you change it, you must change builtin/runtime.go, too.
+// If you change the first four bytes, you must also change the write
+// barrier insertion code.
 var writeBarrier struct {
 	enabled bool    // compiler emits a check of this before calling write barrier
 	pad     [3]byte // compiler uses 32-bit load for "enabled" field