cmd/vet/all: check platforms concurrently
cmd/vet/all: check platforms concurrently

Change-Id: I63e7fd7f62aa80e1252b0c5b6c472439aa66da73
Reviewed-on: https://go-review.googlesource.com/29169
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

derekmarcotte/go,momchil-velikov/go,christopher-henderson/Go,suncycheng/go,ImJasonH/go,goodlang/good,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,polyverse-security/go,ImJasonH/go,suncycheng/go,codestation/go,polyverse-security/go,golang/go,momchil-velikov/go,CAFxX/go,cloudflare/go,goodlang/good,Cofyc/go,linux-on-ibm-z/go,akutz/go,ImJasonH/go,linux-on-ibm-z/go,Cofyc/go,momchil-velikov/go,golang/go,vsekhar/elastic-go,codestation/go,linux-on-ibm-z/go,nawawi/go,vsekhar/elastic-go,Cofyc/go,goodlang/good,cloudflare/go,akutz/go,nawawi/go,polyverse-security/go,golang/go,ImJasonH/go,akutz/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,ImJasonH/go,codestation/go,nawawi/go,nawawi/go,derekmarcotte/go,CAFxX/go,golang/go,goodlang/good,goodlang/good,josharian/go,Cofyc/go,nawawi/go,akutz/go,vsekhar/elastic-go,golang/go,momchil-velikov/go,golang/go,josharian/go,momchil-velikov/go,akutz/go,Cofyc/go,Cofyc/go,derekmarcotte/go,cloudflare/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,akutz/go,suncycheng/go,goodlang/good,josharian/go,polyverse-security/go,Cofyc/go,josharian/go,cloudflare/go,suncycheng/go,polyverse-security/go,linux-on-ibm-z/go,linux-on-ibm-z/go,vsekhar/elastic-go,ImJasonH/go,derekmarcotte/go,derekmarcotte/go,vsekhar/elastic-go,josharian/go,josharian/go,cloudflare/go,akutz/go,momchil-velikov/go,goodlang/good,momchil-velikov/go,CAFxX/go,cloudflare/go,linux-on-ibm-z/go,codestation/go,suncycheng/go,nawawi/go,polyverse-security/go,cloudflare/go,vsekhar/elastic-go,suncycheng/go,vsekhar/elastic-go,ImJasonH/go,josharian/go,CAFxX/go,goodlang/good,codestation/go,suncycheng/go,golang/go,polyverse-security/go,Cofyc/go,ImJasonH/go,nawawi/go,linux-on-ibm-z/go,codestation/go,golang/go,vsekhar/elastic-go,codestation/go,suncycheng/go,derekmarcotte/go,akutz/go,momchil-velikov/go,josharian/go,CAFxX/go