This is vendored copy of the pure-Go version of math/big.
To update, run vendor.bash in place.

This will permit the use of the new big.Float functionality in
gc (which is not available in 1.4, the version used for bootstrapping).

Change-Id: I4dcdea875d54710005ca3fdea2e0e30422b1b46d
Reviewed-on: https://go-review.googlesource.com/7857Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>