go.tools/ssa/interp: fix build (add missing runtime function)
go.tools/ssa/interp: fix build (add missing runtime function)

Introduced by CL 41860043.

R=adonovan
TBR=adonovan
CC=golang-dev
https://codereview.appspot.com/42950044

karmakaze/go.tools,lattera/go.tools,chenjianlong/go.tools,lattera/go.tools,lattera/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools