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/racewalk.go b/src/cmd/internal/gc/racewalk.go
index 3fd2c26..2b3fc30 100644
--- a/src/cmd/internal/gc/racewalk.go
+++ b/src/cmd/internal/gc/racewalk.go
@@ -629,7 +629,7 @@
 	}
 }
 
-// appendinit is like addinit in subr.c
+// appendinit is like addinit in subr.go
 // but appends rather than prepends.
 func appendinit(np **Node, init *NodeList) {
 	if init == nil {