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/reflect/value.go b/src/reflect/value.go
index ad48152..ad106f5 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -176,7 +176,7 @@
 
 // nonEmptyInterface is the header for a interface value with methods.
 type nonEmptyInterface struct {
-	// see ../runtime/iface.c:/Itab
+	// see ../runtime/iface.go:/Itab
 	itab *struct {
 		ityp   *rtype // static interface type
 		typ    *rtype // dynamic concrete type