runtime: clamp OS-reported number of processors to _MaxGomaxprocs
runtime: clamp OS-reported number of processors to _MaxGomaxprocs

So that all Go processes do not die on startup on a system with >256 CPUs.

I tested this by hacking osinit to set ncpu to 1000.

Updates #15131

Change-Id: I52e061a0de97be41d684dd8b748fa9087d6f1aef
Reviewed-on: https://go-review.googlesource.com/21599
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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