go/build: allow @ in #cgo directives for OS X
go/build: allow @ in #cgo directives for OS X

Fixes #13720.

Change-Id: I2e48454696f37db419370630f913590c435cd9f0
Reviewed-on: https://go-review.googlesource.com/18331
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

Samurais/go,vanloswang/go,akutz/go,derekmarcotte/go,akutz/go,cloudflare/go,cyberroadie/go,derekmarcotte/go,cyberroadie/go,Cofyc/go,linux-on-ibm-z/go,theif519/go,nawawi/go,sndnvaps/go,theif519/go,linux-on-ibm-z/go,vanloswang/go,cyberroadie/go,derekmarcotte/go,linux-on-ibm-z/go,golang/go,ganboing/go-esx,vanloswang/go,Endika/go,ganboing/go-esx,mwhudson/go,goodlang/good,theif519/go,vsekhar/elastic-go,momchil-velikov/go,suncycheng/go,Samurais/go,christopher-henderson/Go,ImJasonH/go,miolini/go,nawawi/go,derekmarcotte/go,theif519/go,codestation/go,momchil-velikov/go,cyberroadie/go,linux-on-ibm-z/go,vsekhar/elastic-go,polyverse-security/go,theif519/go,Endika/go,Cofyc/go,mwhudson/go,akutz/go,momchil-velikov/go,binarycrusader/go,cloudflare/go,4ad/go,linux-on-ibm-z/go,binarycrusader/go,ImJasonH/go,vsekhar/elastic-go,momchil-velikov/go,goodlang/good,miolini/go,nawawi/go,mwhudson/go,theif519/go,linux-on-ibm-z/go,cyberroadie/go,goodlang/good,ImJasonH/go,polyverse-security/go,derekmarcotte/go,linux-on-ibm-z/go,CAFxX/go,goodlang/good,vsekhar/elastic-go,akutz/go,vanloswang/go,Endika/go,sndnvaps/go,ImJasonH/go,josharian/go,polyverse-security/go,CAFxX/go,mwhudson/go,vsekhar/elastic-go,linux-on-ibm-z/go,miolini/go,vanloswang/go,ganboing/go-esx,binarycrusader/go,vanloswang/go,cloudflare/go,nawawi/go,sndnvaps/go,binarycrusader/go,4ad/go,binarycrusader/go,goodlang/good,sndnvaps/go,cyberroadie/go,theif519/go,golang/go,momchil-velikov/go,Samurais/go,polyverse-security/go,suncycheng/go,vanloswang/go,akutz/go,mwhudson/go,4ad/go,josharian/go,codestation/go,codestation/go,polyverse-security/go,vsekhar/elastic-go,miolini/go,cloudflare/go,ImJasonH/go,4ad/go,sndnvaps/go,golang/go,sndnvaps/go,akutz/go,vsekhar/elastic-go,binarycrusader/go,Cofyc/go,akutz/go,akutz/go,codestation/go,miolini/go,ImJasonH/go,Samurais/go,derekmarcotte/go,Endika/go,nawawi/go,ganboing/go-esx,suncycheng/go,ImJasonH/go,4ad/go,Samurais/go,miolini/go,CAFxX/go,Endika/go,suncycheng/go,cyberroadie/go,Endika/go,Cofyc/go,momchil-velikov/go,momchil-velikov/go,cyberroadie/go,suncycheng/go,cloudflare/go,goodlang/good,josharian/go,binarycrusader/go,codestation/go,Cofyc/go,Cofyc/go,sndnvaps/go,josharian/go,derekmarcotte/go,golang/go,Cofyc/go,ImJasonH/go,sndnvaps/go,Endika/go,Samurais/go,josharian/go,Samurais/go,cloudflare/go,polyverse-security/go,nawawi/go,cloudflare/go,suncycheng/go,miolini/go,4ad/go,ganboing/go-esx,josharian/go,cloudflare/go,josharian/go,CAFxX/go,Endika/go,suncycheng/go,derekmarcotte/go,goodlang/good,nawawi/go,vanloswang/go,4ad/go,codestation/go,codestation/go,golang/go,momchil-velikov/go,mwhudson/go,Samurais/go,mwhudson/go,miolini/go,polyverse-security/go,golang/go,binarycrusader/go,nawawi/go,ganboing/go-esx,CAFxX/go,ganboing/go-esx,suncycheng/go,goodlang/good,polyverse-security/go,golang/go,4ad/go,josharian/go,vsekhar/elastic-go,CAFxX/go,CAFxX/go,ganboing/go-esx,Cofyc/go,CAFxX/go,codestation/go,theif519/go,mwhudson/go,golang/go