internal/poll: define PollDescriptor on plan9
internal/poll: define PollDescriptor on plan9

Fixes #19114.

Change-Id: I352add53d6ee8bf78792564225099f8537ac6b46
Reviewed-on: https://go-review.googlesource.com/37106
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>

Cofyc/go,Cofyc/go,vsekhar/elastic-go,polyverse-security/go,josharian/go,derekmarcotte/go,golang/go,ImJasonH/go,CAFxX/go,ImJasonH/go,ImJasonH/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,vsekhar/elastic-go,nawawi/go,polyverse-security/go,golang/go,Cofyc/go,cloudflare/go,vsekhar/elastic-go,linux-on-ibm-z/go,nawawi/go,cloudflare/go,ImJasonH/go,ImJasonH/go,polyverse-security/go,derekmarcotte/go,codestation/go,codestation/go,linux-on-ibm-z/go,josharian/go,cloudflare/go,ImJasonH/go,cloudflare/go,derekmarcotte/go,cloudflare/go,codestation/go,josharian/go,christopher-henderson/Go,CAFxX/go,vsekhar/elastic-go,codestation/go,akutz/go,nawawi/go,Cofyc/go,nawawi/go,polyverse-security/go,codestation/go,golang/go,golang/go,akutz/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,polyverse-security/go,akutz/go,Cofyc/go,akutz/go,golang/go,cloudflare/go,akutz/go,linux-on-ibm-z/go,ImJasonH/go,derekmarcotte/go,akutz/go,vsekhar/elastic-go,Cofyc/go,Cofyc/go,josharian/go,golang/go,derekmarcotte/go,CAFxX/go,polyverse-security/go,linux-on-ibm-z/go,CAFxX/go,derekmarcotte/go,nawawi/go,CAFxX/go,vsekhar/elastic-go,golang/go,linux-on-ibm-z/go,josharian/go,polyverse-security/go,akutz/go,linux-on-ibm-z/go,CAFxX/go,linux-on-ibm-z/go,vsekhar/elastic-go,golang/go,CAFxX/go,codestation/go,ImJasonH/go,derekmarcotte/go,akutz/go,cloudflare/go,nawawi/go,codestation/go,CAFxX/go,josharian/go,josharian/go,Cofyc/go,cloudflare/go,nawawi/go,nawawi/go