cmd/cgo: use "!compiler_bootstrap" tag instead of "go1.18"

The illumos builder is using a Go 1.18 prerelease toolchain for
bootstrapping, which doesn't have ast.IndexListExpr. But we can
instead check for the "compiler_bootstrap" build tag that's already
used for distinguishing binaries built for toolchain1.

Fixes #48863.

Change-Id: I87b672322eee22a60ab8d0bb3be5f76cffc97545
Reviewed-on: https://go-review.googlesource.com/c/go/+/354695


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>