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://golang.org/cl/93410045

sshetty10/tools,syreclabs/go-tools,cmars/tools,argandas/tools,maddyonline/tools,nicot/tools,syreclabs/go-tools,runcom/tools,4ad/tools,josharian/tools,runcom/tools,jdhenke/tools,sshetty10/tools,springning/tools,glasser/golang-tools,cw2018/tools,matryer/tools,argandas/tools,springning/tools,jdhenke/tools,springning/tools,josharian/tools,cw2018/tools,cmars/tools,maddyonline/tools,syreclabs/go-tools,golang/tools,magiccao/tools,golang/tools,cmars/tools,4ad/tools,magiccao/tools,dmitris/tools,syreclabs/go-tools,magiccao/tools,sshetty10/tools,syreclabs/go-tools,argandas/tools,matryer/tools,4ad/tools,golang/tools,josharian/tools,nicot/tools,matryer/tools,springning/tools,cw2018/tools,matryer/tools,syreclabs/go-tools,argandas/tools,golang/tools,nicot/tools,dmitris/tools,dmitris/tools,runcom/tools,glasser/golang-tools,cw2018/tools,jdhenke/tools,dmitris/tools,magiccao/tools,runcom/tools,cmars/tools,sshetty10/tools,golang/tools,jdhenke/tools,maddyonline/tools,nicot/tools,glasser/golang-tools,glasser/golang-tools,maddyonline/tools,josharian/tools