cmd/dist, cmd/internal/gc: switch gc from math/big to vendored math/big

Change-Id: Iab9a1f814acd53d5707bfcd6c3f9616c79fa7789
Reviewed-on: https://go-review.googlesource.com/7858
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/buildtool.go

@@ -39,6 +39,7 @@ var bootstrapDirs = []string{
 	"asm/internal/flags",
 	"asm/internal/lex",
 	"internal/asm",
+	"internal/gc/big",
 	"internal/gc",
 	"internal/ld",
 	"internal/obj",

src/cmd/internal/gc/go.go

@@ -6,8 +6,8 @@ package gc
 
 import (
 	"bytes"
+	"cmd/internal/gc/big"
 	"cmd/internal/obj"
-	"math/big"
 )
 
 // avoid <ctype.h>

src/cmd/internal/gc/mparith1.go

@@ -5,10 +5,10 @@
 package gc
 
 import (
+	"cmd/internal/gc/big"
 	"cmd/internal/obj"
 	"fmt"
 	"math"
-	"math/big"
 )
 
 /// uses arithmetic