runtime: move mksizeclasses.go to runtime/_mkmalloc

This will allow us to share code with the specialized malloc code
generator.

Change-Id: I6a6a696450a5039a957811fb06228122d494ddce
Reviewed-on: https://go-review.googlesource.com/c/go/+/700495
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>

src/internal/runtime/gc/sizeclasses.go

@@ -1,5 +1,5 @@
 // Code generated by mksizeclasses.go; DO NOT EDIT.
-//go:generate go run mksizeclasses.go
+//go:generate go -C ../../../runtime/_mkmalloc run mksizeclasses.go
 
 package gc
 

src/runtime/_mkmalloc/mksizeclasses.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build ignore
-
 // Generate tables for small malloc size classes.
 //
 // See malloc.go for overview.
@@ -41,7 +39,7 @@ import (
 	"os"
 )
 
-// Generate msize.go
+// Generate internal/runtime/gc/msize.go
 
 var stdout = flag.Bool("stdout", false, "write to stdout instead of sizeclasses.go")
 
@@ -50,7 +48,7 @@ func main() {
 
 	var b bytes.Buffer
 	fmt.Fprintln(&b, "// Code generated by mksizeclasses.go; DO NOT EDIT.")
-	fmt.Fprintln(&b, "//go:generate go run mksizeclasses.go")
+	fmt.Fprintln(&b, "//go:generate go -C ../../../runtime/_mkmalloc run mksizeclasses.go")
 	fmt.Fprintln(&b)
 	fmt.Fprintln(&b, "package gc")
 	classes := makeClasses()
@@ -66,7 +64,7 @@ func main() {
 	if *stdout {
 		_, err = os.Stdout.Write(out)
 	} else {
-		err = os.WriteFile("sizeclasses.go", out, 0666)
+		err = os.WriteFile("../../internal/runtime/gc/sizeclasses.go", out, 0666)
 	}
 	if err != nil {
 		log.Fatal(err)