cmd/vet: add amd64p32 (nacl) support to asmdecl check
cmd/vet: add amd64p32 (nacl) support to asmdecl check

Also add 'ErrorString', for checking Plan 9's package syscall.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/93410045

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