cmd/compile: bring vendored copy of math/big up-to-date

cmd/compile: bring vendored copy of math/big up-to-date

These files were not added to the repo. They contain conversion
routines and corresponding tests not used by the compiler and
thus are technically not needed.

However, future changes to math/big (and corresponding updates
of this vendored version) may require these files to exist.
Add them to avoid unnecessary confusion.

Change-Id: Ie390fb54f499463b2bba2fdc084967539afbeeb3
Reviewed-on: https://go-review.googlesource.com/19730
Reviewed-by: Alan Donovan <adonovan@google.com>