key32 is called between entersyscallblock and exitsyscall
stack split may occur if disable inlining and the G is preempted

Fix the problem by describing key32 as nosplit function

Fixes #20510

Change-Id: I1f0787995936f34ef0052cf79fde036f1b338865
Reviewed-on: https://go-review.googlesource.com/44390Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>