cmd/compile, runtime: make room for rangefunc defers

cmd/compile, runtime: make room for rangefunc defers

This is subtle and the compiler and runtime be in sync.
It is easier to develop the rest of the changes (especially when using
toolstash save/restore) if this change is separated out and done first.

Preparation for proposal #61405. The actual logic in the
compiler will be guarded by a GOEXPERIMENT, but it is
easier not to have GOEXPERIMENT-specific data structures
in the runtime, so just make the field always.

Change-Id: I7ec7049b99ae98bf0db365d42966baeec56e3774
Reviewed-on: https://go-review.googlesource.com/c/go/+/510539
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>