cmd/compile: don't emit stack maps for write barrier calls

These are necessarily deeply non-preemptible, so there's no point in
emitting stack maps for them. We already mark them as unsafe points,
so this only affects the runtime, since user code does not emit stack
maps at unsafe points. SSAGenState.PrepareCall also excludes them when
it's sanity checking call stack maps.

Right now this only drops a handful of unnecessary stack maps from the
runtime, but we're about to start emitting stack maps only at calls
for user code, too. At that point, this will matter much more.

For #36365.

Change-Id: Ib3abfedfddc8e724d933a064fa4d573500627990
Reviewed-on: https://go-review.googlesource.com/c/go/+/230542


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>