runtime: fix stack split at bad time when disable inlining

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>

src/runtime/lock_futex.go

@@ -38,6 +38,7 @@ const (
 // affect mutex's state.
 
 // We use the uintptr mutex.key and note.key as a uint32.
+//go:nosplit
 func key32(p *uintptr) *uint32 {
 	return (*uint32)(unsafe.Pointer(p))
 }