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

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