cmd/link: concurrent obj copy for external linking
cmd/link: concurrent obj copy for external linking

Change-Id: I630ae29ecb39252642883398cc51d49133c6f3d7
Reviewed-on: https://go-review.googlesource.com/16451
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,goodlang/good,odeke-em/go,Jordanzuo/go,vanloswang/go,sndnvaps/go,Jordanzuo/go,theif519/go,odeke-em/go,momchil-velikov/go,ImJasonH/go,4ad/go,ganboing/go-esx,theif519/go,Jordanzuo/go,momchil-velikov/go,linux-on-ibm-z/go,derekmarcotte/go,binarycrusader/go,golang/go,deft-code/go,deft-code/go,kim-racktop/go,vsekhar/elastic-go,theif519/go,Samurais/go,fjballest/golang,fjballest/golang,vanloswang/go,Samurais/go,codestation/go,mwhudson/go,4ad/go,derekmarcotte/go,4ad/go,josharian/go,goodlang/good,golang/go,goodlang/good,cyberroadie/go,cyberroadie/go,deft-code/go,akutz/go,kim-racktop/go,deft-code/go,vsekhar/elastic-go,Endika/go,miolini/go,josharian/go,roth1002/go,deft-code/go,ImJasonH/go,kim-racktop/go,malvira/go,cyberroadie/go,miolini/go,cloudflare/go,sndnvaps/go,vanloswang/go,suncycheng/go,golang/go,josharian/go,derekmarcotte/go,polyverse-security/go,momchil-velikov/go,malvira/go,miolini/go,linux-on-ibm-z/go,akutz/go,theif519/go,deft-code/go,golang/go,akutz/go,roth1002/go,akutz/go,theif519/go,4ad/go,vanloswang/go,deft-code/go,miolini/go,ganboing/go-esx,fjballest/golang,ganboing/go-esx,Jordanzuo/go,odeke-em/go,CAFxX/go,binarycrusader/go,josharian/go,codestation/go,codestation/go,akutz/go,suncycheng/go,binarycrusader/go,rachtsingh/gogojuice,CAFxX/go,theif519/go,nawawi/go,Cofyc/go,polyverse-security/go,mwhudson/go,rachtsingh/gogojuice,nawawi/go,Cofyc/go,Endika/go,polyverse-security/go,Endika/go,4ad/go,kim-racktop/go,Jordanzuo/go,kim-racktop/go,nawawi/go,Endika/go,rachtsingh/gogojuice,mwhudson/go,malvira/go,golang/go,codestation/go,vsekhar/elastic-go,malvira/go,mwhudson/go,fjballest/golang,codestation/go,polyverse-security/go,vsekhar/elastic-go,suncycheng/go,Samurais/go,odeke-em/go,josharian/go,fjballest/golang,suncycheng/go,Cofyc/go,malvira/go,miolini/go,fjballest/golang,golang/go,Cofyc/go,4ad/go,ImJasonH/go,linux-on-ibm-z/go,Samurais/go,mwhudson/go,rachtsingh/gogojuice,josharian/go,Endika/go,nawawi/go,4ad/go,derekmarcotte/go,mwhudson/go,codestation/go,Endika/go,vanloswang/go,odeke-em/go,polyverse-security/go,CAFxX/go,sndnvaps/go,malvira/go,linux-on-ibm-z/go,Cofyc/go,Jordanzuo/go,vanloswang/go,fjballest/golang,miolini/go,polyverse-security/go,vsekhar/elastic-go,vanloswang/go,Samurais/go,binarycrusader/go,cloudflare/go,sndnvaps/go,goodlang/good,rachtsingh/gogojuice,goodlang/good,josharian/go,odeke-em/go,momchil-velikov/go,ganboing/go-esx,ImJasonH/go,codestation/go,malvira/go,mwhudson/go,ganboing/go-esx,Cofyc/go,momchil-velikov/go,sndnvaps/go,CAFxX/go,4ad/go,Samurais/go,suncycheng/go,goodlang/good,suncycheng/go,vanloswang/go,golang/go,ganboing/go-esx,theif519/go,Cofyc/go,linux-on-ibm-z/go,nawawi/go,kim-racktop/go,christopher-henderson/Go,cloudflare/go,malvira/go,fjballest/golang,odeke-em/go,polyverse-security/go,ImJasonH/go,deft-code/go,linux-on-ibm-z/go,golang/go,Endika/go,akutz/go,Samurais/go,momchil-velikov/go,derekmarcotte/go,binarycrusader/go,polyverse-security/go,roth1002/go,rachtsingh/gogojuice,rachtsingh/gogojuice,cyberroadie/go,roth1002/go,theif519/go,nawawi/go,CAFxX/go,suncycheng/go,mwhudson/go,roth1002/go,sndnvaps/go,derekmarcotte/go,Cofyc/go,CAFxX/go,Endika/go,josharian/go,ImJasonH/go,CAFxX/go,miolini/go,cyberroadie/go,cloudflare/go,sndnvaps/go,cyberroadie/go,ImJasonH/go,derekmarcotte/go,linux-on-ibm-z/go,kim-racktop/go,akutz/go,vsekhar/elastic-go,suncycheng/go,roth1002/go,Samurais/go,cloudflare/go,ganboing/go-esx,nawawi/go,nawawi/go,vsekhar/elastic-go,miolini/go,codestation/go,cloudflare/go,cloudflare/go,Jordanzuo/go,momchil-velikov/go,cyberroadie/go,sndnvaps/go,linux-on-ibm-z/go,binarycrusader/go,cyberroadie/go,binarycrusader/go,goodlang/good,Jordanzuo/go,kim-racktop/go,vsekhar/elastic-go,momchil-velikov/go,goodlang/good,cloudflare/go,akutz/go,roth1002/go,derekmarcotte/go,binarycrusader/go,rachtsingh/gogojuice,odeke-em/go,roth1002/go,ganboing/go-esx,ImJasonH/go