runtime: avoid gp.lockedm race in exitsyscall0

Following https://golang.org/cl/291329, exitsyscall0 accesses gp.lockedm
after releasing gp to the global runq. This creates a race window where
another M may schedule the (unlocked) G, which subsequently calls
LockOSThread, setting gp.lockedm and thus causing exitsyscall0 to think
it should call stoplockedm.

Avoid this race by checking if gp is locked before releasing it to the
global runq.

Fixes #46524

Change-Id: I3acdaf09e7a2178725adbe61e985130e9ebd0680
Reviewed-on: https://go-review.googlesource.com/c/go/+/324350


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>