cmd/link: fix left-over reference to reflect.c

Replaced ../gc/reflect.c with
cmd/compile/internal/gc/reflect.go.

Fixes #20525

Change-Id: Ibafd36ea446ace5c677df27873a4bbf716a0a9bc
Reviewed-on: https://go-review.googlesource.com/44376Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/decodesym.go

@@ -14,7 +14,7 @@ import (
 
 // Decoding the type.* symbols.	 This has to be in sync with
 // ../../runtime/type.go, or more specifically, with what
-// ../gc/reflect.c stuffs in these.
+// cmd/compile/internal/gc/reflect.go stuffs in these.
 
 // tflag is documented in reflect/type.go.
 //