[release-branch.go1.15] runtime: wait for preemption signals before syscall.Exec

For #41702
For #41704
For #42023

Change-Id: If07f40b1d73b8f276ee28ffb8b7214175e56c24d
Reviewed-on: https://go-review.googlesource.com/c/go/+/262817


Trust: Ian Lance Taylor <iant@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit 05739d6f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/264022