runtime: eliminate write barriers from save
runtime: eliminate write barriers from save

As for dropg, save is writing a nil pointer that will generate a write
barrier with the hybrid barrier. However, in this case, ctxt always
should already be nil, so replace the write with an assertion that
this is the case.

At this point, we're ready to disable the write barrier elision
optimizations that interfere with the hybrid barrier.

Updates #17503.

Change-Id: I83208e65aa33403d442401f355b2e013ab9a50e9
Reviewed-on: https://go-review.googlesource.com/31571
Reviewed-by: Rick Hudson <838bd060584c511f21c75c69ea04b7b120d86af0@golang.org>

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