cmd/go: don't fail on invalid GOOS/GOARCH pair when using gccgo
cmd/go: don't fail on invalid GOOS/GOARCH pair when using gccgo

Fixes #12272

Change-Id: I0306ce0ef4a87df2158df3b7d4d8d93a1cb6dabc
Reviewed-on: https://go-review.googlesource.com/24864
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

Cofyc/go,goodlang/good,akutz/go,josharian/go,polyverse-security/go,goodlang/good,christopher-henderson/Go,suncycheng/go,ImJasonH/go,linux-on-ibm-z/go,Cofyc/go,suncycheng/go,akutz/go,golang/go,golang/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,josharian/go,4ad/go,josharian/go,nawawi/go,nawawi/go,goodlang/good,vsekhar/elastic-go,momchil-velikov/go,suncycheng/go,4ad/go,goodlang/good,linux-on-ibm-z/go,cloudflare/go,4ad/go,golang/go,ImJasonH/go,polyverse-security/go,Cofyc/go,linux-on-ibm-z/go,derekmarcotte/go,momchil-velikov/go,codestation/go,nawawi/go,ImJasonH/go,linux-on-ibm-z/go,vsekhar/elastic-go,CAFxX/go,CAFxX/go,derekmarcotte/go,momchil-velikov/go,codestation/go,akutz/go,derekmarcotte/go,CAFxX/go,Cofyc/go,akutz/go,cloudflare/go,josharian/go,nawawi/go,ImJasonH/go,derekmarcotte/go,cloudflare/go,golang/go,codestation/go,nawawi/go,josharian/go,goodlang/good,ImJasonH/go,nawawi/go,linux-on-ibm-z/go,golang/go,suncycheng/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,momchil-velikov/go,linux-on-ibm-z/go,CAFxX/go,codestation/go,4ad/go,vsekhar/elastic-go,josharian/go,codestation/go,Cofyc/go,ImJasonH/go,4ad/go,vsekhar/elastic-go,vsekhar/elastic-go,momchil-velikov/go,josharian/go,Cofyc/go,nawawi/go,polyverse-security/go,akutz/go,momchil-velikov/go,derekmarcotte/go,Cofyc/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,CAFxX/go,codestation/go,4ad/go,josharian/go,cloudflare/go,momchil-velikov/go,derekmarcotte/go,goodlang/good,polyverse-security/go,4ad/go,derekmarcotte/go,golang/go,codestation/go,vsekhar/elastic-go,suncycheng/go,goodlang/good,momchil-velikov/go,suncycheng/go,goodlang/good,cloudflare/go,akutz/go,vsekhar/elastic-go,polyverse-security/go,golang/go,codestation/go,polyverse-security/go,suncycheng/go,ImJasonH/go,CAFxX/go,vsekhar/elastic-go,cloudflare/go,suncycheng/go,linux-on-ibm-z/go,nawawi/go,CAFxX/go,4ad/go,golang/go,polyverse-security/go