go.tools/ssa/interp: enable tests of builtin(f()) where f has multiple results.
go.tools/ssa/interp: enable tests of builtin(f()) where f has multiple results.

R=gri
CC=golang-dev
https://codereview.appspot.com/14408043

d0f/go-zh.tools,8limbs/gotools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools