runtime: move tSweepTerm capture closer to STW
runtime: move tSweepTerm capture closer to STW

tSweepTerm and pauseStart are supposed to be when STW was triggered,
but right now they're captured a bit before STW. Move these down to
immediately before we trigger STW.

Fixes #19590.

Change-Id: Icd48a5c4d45c9b36187ff986e4f178b5064556c1
Reviewed-on: https://go-review.googlesource.com/49612
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

josharian/go,polyverse-security/go,codestation/go,nawawi/go,codestation/go,akutz/go,codestation/go,codestation/go,vsekhar/elastic-go,nawawi/go,Cofyc/go,CAFxX/go,golang/go,josharian/go,polyverse-security/go,golang/go,codestation/go,Cofyc/go,CAFxX/go,nawawi/go,vsekhar/elastic-go,josharian/go,akutz/go,cloudflare/go,akutz/go,golang/go,Cofyc/go,cloudflare/go,Cofyc/go,cloudflare/go,josharian/go,cloudflare/go,codestation/go,codestation/go,CAFxX/go,cloudflare/go,josharian/go,Cofyc/go,josharian/go,golang/go,nawawi/go,CAFxX/go,josharian/go,vsekhar/elastic-go,CAFxX/go,Cofyc/go,akutz/go,cloudflare/go,akutz/go,Cofyc/go,polyverse-security/go,polyverse-security/go,golang/go,polyverse-security/go,nawawi/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,CAFxX/go,vsekhar/elastic-go,vsekhar/elastic-go,josharian/go,codestation/go,nawawi/go,polyverse-security/go,golang/go,CAFxX/go,akutz/go,Cofyc/go,cloudflare/go,golang/go,nawawi/go,vsekhar/elastic-go,nawawi/go,akutz/go,christopher-henderson/Go,CAFxX/go,golang/go,polyverse-security/go,vsekhar/elastic-go,cloudflare/go