[dev.typeparams] cmd/compile: skip types2 GC test during bootstrapping

Unified includes a check to make sure that types2 memory has been
garbage collected, but it relies on precise finalization, which we
provide (for dynamically allocated objects, at least) but isn't
guaranteed by the Go spec. In particular, Go 1.4 doesn't provide this.

The check is strictly unnecessary and only exists to make sure we
don't regress and start holding onto types2 memory accidentally. So
just disable the check during bootstrap builds.

Change-Id: Ie54fe53b2edba02c0b0b1e5ae39d81be8a0ace8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/329269


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>