[release-branch.go1.17] cmd/internal/obj/x86: modify the threshold of assert loop for span6

Updates #49716.
Fixes #50942.

Change-Id: I7ed73f874c2ee1ee3f31c9c4428ed484167ca803
Reviewed-on: https://go-review.googlesource.com/c/go/+/366094


Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Heschi Kreinick <heschi@google.com>
(cherry picked from commit 14f2b2a4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/383214


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>