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/select.go b/src/cmd/internal/gc/select.go
index 5816428..e3c9294 100644
--- a/src/cmd/internal/gc/select.go
+++ b/src/cmd/internal/gc/select.go
@@ -324,7 +324,7 @@
 	lineno = int32(lno)
 }
 
-// Keep in sync with src/runtime/chan.h.
+// Keep in sync with src/runtime/runtime2.go and src/runtime/select.go.
 func selecttype(size int32) *Type {
 	// TODO(dvyukov): it's possible to generate SudoG and Scase only once
 	// and then cache; and also cache Select per size.