runtime: only permit SetCgoTraceback to be called once
runtime: only permit SetCgoTraceback to be called once

Accept a duplicate call, but nothing else.

Change-Id: Iec24bf5ddc3b0f0c559ad2158339aca698601743
Reviewed-on: https://go-review.googlesource.com/23692
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

ImJasonH/go,josharian/go,polyverse-security/go,goodlang/good,nawawi/go,derekmarcotte/go,akutz/go,nawawi/go,akutz/go,theif519/go,ImJasonH/go,linux-on-ibm-z/go,vsekhar/elastic-go,4ad/go,polyverse-security/go,codestation/go,vsekhar/elastic-go,akutz/go,momchil-velikov/go,golang/go,linux-on-ibm-z/go,Cofyc/go,suncycheng/go,suncycheng/go,theif519/go,polyverse-security/go,momchil-velikov/go,cloudflare/go,polyverse-security/go,momchil-velikov/go,theif519/go,vsekhar/elastic-go,derekmarcotte/go,ImJasonH/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,Cofyc/go,suncycheng/go,CAFxX/go,4ad/go,goodlang/good,CAFxX/go,cloudflare/go,linux-on-ibm-z/go,linux-on-ibm-z/go,vsekhar/elastic-go,CAFxX/go,josharian/go,goodlang/good,vsekhar/elastic-go,suncycheng/go,polyverse-security/go,golang/go,ImJasonH/go,josharian/go,akutz/go,golang/go,Cofyc/go,Cofyc/go,derekmarcotte/go,goodlang/good,CAFxX/go,josharian/go,CAFxX/go,suncycheng/go,4ad/go,momchil-velikov/go,ImJasonH/go,codestation/go,4ad/go,momchil-velikov/go,derekmarcotte/go,akutz/go,cloudflare/go,codestation/go,goodlang/good,ImJasonH/go,christopher-henderson/Go,derekmarcotte/go,josharian/go,momchil-velikov/go,cloudflare/go,vsekhar/elastic-go,4ad/go,codestation/go,theif519/go,polyverse-security/go,theif519/go,josharian/go,derekmarcotte/go,momchil-velikov/go,codestation/go,cloudflare/go,josharian/go,goodlang/good,codestation/go,polyverse-security/go,suncycheng/go,Cofyc/go,vsekhar/elastic-go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,goodlang/good,linux-on-ibm-z/go,vsekhar/elastic-go,4ad/go,golang/go,ImJasonH/go,akutz/go,linux-on-ibm-z/go,Cofyc/go,akutz/go,theif519/go,theif519/go,codestation/go,CAFxX/go,4ad/go,nawawi/go,4ad/go,ImJasonH/go,Cofyc/go,cloudflare/go,codestation/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,golang/go,akutz/go,nawawi/go,nawawi/go,josharian/go,suncycheng/go,momchil-velikov/go,golang/go,theif519/go,golang/go,nawawi/go,suncycheng/go,derekmarcotte/go,goodlang/good,linux-on-ibm-z/go