cmd/dist: copy needed packages from standard library during bootstrap

This allows use of newer math/big (and later debug/pe)
without maintaining a vendored copy somewhere in cmd.

Use for math/big, deleting cmd/compile/internal/big.

Change-Id: I2bffa7a9ef115015be29fafdb02acc3e7a665d11
Reviewed-on: https://go-review.googlesource.com/31010
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -10,7 +10,7 @@ package gc
 
 import (
 	"bufio"
-	"cmd/compile/internal/big"
+	"math/big"
 	"encoding/binary"
 	"fmt"
 	"strconv"