runtime: use correct MOV for plan9 brk_ ret value
runtime: use correct MOV for plan9 brk_ ret value

Updates #11041

Change-Id: I78f8d48f00cfbb451e37c868cc472ef06ea0fd95
Reviewed-on: https://go-review.googlesource.com/27491
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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