cmd/compile/internal/big: update and apply vendor.bash

cmd/compile/internal/big: update and apply vendor.bash

Package-external tests must use the vendored math/big package, not
the original one, otherwise tests may fail if there are discrepancies
in the implementation.

Change-Id: Ic5f0489aa6420ffea1f488633453f871ce1f0f66
Reviewed-on: https://go-review.googlesource.com/10380
Reviewed-by: Ian Lance Taylor <iant@golang.org>