cmd/5g: avoid clash between R13 and F3 registers.

Fixes #6247.

R=golang-dev, lucio.dere, bradfitz
CC=golang-dev
https://golang.org/cl/13216043

src/cmd/5g/gg.h

@@ -51,7 +51,7 @@ struct	Prog
 
 #define REGALLOC_R0 0
 #define REGALLOC_RMAX REGEXT
-#define REGALLOC_F0 (REGALLOC_RMAX+1)
+#define REGALLOC_F0 NREG
 #define REGALLOC_FMAX (REGALLOC_F0 + FREGEXT)
 
 EXTERN	int32	dynloc;

test/fixedbugs/issue6247.go

+// compile
+
+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 6247: 5g used to be confused by the numbering
+// of floating-point registers.
+
+package main
+
+var p map[string]interface{}
+var v interface{}
+
+func F() {
+	p["hello"] = v.(complex128) * v.(complex128)
+}