go.tools/playground/socket: fix nacl/arm command line. nacl_helper_bootstrap_arm doesn't consult $PATH for sel_ldr_arm.
go.tools/playground/socket: fix nacl/arm command line.
nacl_helper_bootstrap_arm doesn't consult $PATH for sel_ldr_arm.

LGTM=adg
R=adg, dan.kortschak
CC=golang-codereviews
https://codereview.appspot.com/112010044

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