exp/gotype: fix build breakage due to https://code.google.com/p/go/source/detail?r=ca5e5de48173

Add 'math/big' to blacklist of packages that use shift
operations as yet unsupported by go/types.

(The failure was masked due to local bugfixes in my client.)

R=rsc, bradfitz, bradfitz
CC=golang-dev
https://codereview.appspot.com/7220057