cmd/compile: dump stack trace in Fatalf during development

See discussion in #15029.

Change-Id: I5cc8be5737ddb7c1f5e4a6cd92cf557af45e961d
Reviewed-on: https://go-review.googlesource.com/21347
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/subr.go

@@ -11,6 +11,7 @@ import (
 	"encoding/binary"
 	"fmt"
 	"os"
+	"runtime/debug"
 	"sort"
 	"strconv"
 	"strings"
@@ -151,6 +152,11 @@ func Fatalf(fmt_ string, args ...interface{}) {
 		fmt.Printf("\n")
 		fmt.Printf("Please file a bug report including a short program that triggers the error.\n")
 		fmt.Printf("https://golang.org/issue/new\n")
+	} else {
+		// Not a release; dump a stack trace, too.
+		fmt.Println()
+		os.Stdout.Write(debug.Stack())
+		fmt.Println()
 	}
 
 	hcrash()