runtime,reflect,cmd/internal/gc: Fix comments referring to .c/.h files

Everything has moved to Go, but comments still refer to .c/.h files.
Fix all of those up, at least for these three directories.

Fixes #10138

Change-Id: Ie5efe89b247841e0b3f82aac5256b2c606ef67dc
Reviewed-on: https://go-review.googlesource.com/7431
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/src/cmd/internal/gc/range.go b/src/cmd/internal/gc/range.go
index 8dbb002..ca901d2 100644
--- a/src/cmd/internal/gc/range.go
+++ b/src/cmd/internal/gc/range.go
@@ -306,7 +306,7 @@
 		hit := n.Alloc
 		hit.Type = th
 		n.Left = nil
-		keyname := newname(th.Type.Sym)      // depends on layout of iterator struct.  See reflect.c:hiter
+		keyname := newname(th.Type.Sym)      // depends on layout of iterator struct.  See reflect.go:hiter
 		valname := newname(th.Type.Down.Sym) // ditto
 
 		fn := syslook("mapiterinit", 1)