go.tools/ssa/interp: set GOARCH to runtime.GOARCH in target program.
go.tools/ssa/interp: set GOARCH to runtime.GOARCH in target program.

Causes $GOROOT/test/env.go to pass if GOARCH is unset in parent's env.

R=gri
CC=golang-dev
https://codereview.appspot.com/13709043

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