[roll] Roll fuchsia [kernel][policy] Change EnforceBasicPolicy to use thread signals

Prior to this change EnforceBasicPolicy might directly call
ProcessDispatcher::Kill, which acquires one or more mutexes, which
meant it could not be used in contexts where blocking was not allowed
(e.g. while holding a spinlock).

This commit changes EnforceBasicPolicy to use the thread signal
infrastructure to call Kill at a "safe-point".  Now,
EnforceBasicPolicy can be called even when holding a spinlock.

Add a new thread signal, THREAD_SIGNAL_POLICY_KILL, similar to
THREAD_SIGNAL_POLICY_EXCEPTION.  EnforceBasicPolicy now raise this
signal.  Once the thread has returned out to a "safe-point",
ProcessPendingSignals will terminate the process.

Original-Original-Bug: b/511571339
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1618269
Original-Original-Revision: 6df812a3a5a16c7f2d8a8fb3fad1379303ae459e
GitOrigin-RevId: 124b717ef48a0203ca02ecb4993dd4c9774301af
Change-Id: I700d03ebc1b15e0c3596d0f1faf7ae032a4f2a6f
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1620480
Cr-Commit-Position: refs/heads/main@{#193297}