runtime: skip tracing events that would cause reentrancy

Some of the new experimental events added have a problem in that they
might be emitted during stack growth. This is, to my knowledge, the only
restriction on the tracer, because the tracer otherwise prevents
preemption, avoids allocation, and avoids write barriers. However, the
stack can grow from within the tracer. This leads to
tracing-during-tracing which can result in lost buffers and broken event
streams. (There's a debug mode to get a nice error message, but it's
disabled by default.)

This change resolves the problem by skipping writing out these new
events. This results in the new events sometimes being broken (alloc
without a free, free without an alloc) but for now that's OK. Before the
freeze begins we just want to fix broken tests; tools interpreting these
events will be totally in-house to begin with, and if they have to be a
little bit smarter about missing information, that's OK. In the future
we'll have a more robust fix for this, but it appears that it's going to
require making the tracer fully reentrant. (This is not too hard; either
we force flushing all buffers when going reentrant (which is actually
somewhat subtle with respect to event ordering) or we isolate down just
the actual event writing to be atomic with respect to stack growth. Both
are just bigger changes on shared codepaths that are scary to land this
late in the release cycle.)

Fixes #67379.

Change-Id: I46bb7e470e61c64ff54ac5aec5554b828c1ca4be
Reviewed-on: https://go-review.googlesource.com/c/go/+/587597
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>