cmd/cgo/internal/test: build error on macOS 26 in Go 1.25

[dmitshur]

The recently added darwin/arm64 builder with macOS 26 (issue #76798) uncovers a failing test on release-branch.go1.25:

I can reproduce locally with:

# (with macOS 26)
$ GOTOOLCHAIN=go1.25.8 go build cmd/cgo/internal/test
go: downloading go1.25.8 (darwin/arm64)
# cmd/cgo/internal/test [cmd/cgo/internal/test.test]
.../cmd/cgo/internal/test/test.go:250:16: error: variable length array folded to constant array as an extension [-Werror,-Wgnu-folding-constant]

On macOS 15, it works fine, on macOS 26 it fails as above.

Doesn't happen with Go 1.26.1 or tip.

CC @golang/darwin, @prattmic.

[cherrymui]

http://go.dev/cl/692877
Fixed when I tested Xcode 26 beta.

[dmitshur]

Thanks. Since this is a test-only fix, and should be safe, we can probably backport it. (Another option would be to skip the test, but that doesn't seem to be needed.)

@gopherbot Please consider this for backport to Go 1.25. It's needed to fix a test that otherwise fails on macOS 26, and get the recently added builder green on release branch.go1.25.

[gopherbot]

Backport issue(s) opened: #77999 (for 1.25).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[dmitshur]

Closing as fixed in Go 1.26 by CL 692877.

[gopherbot]

Change https://go.dev/cl/752520 mentions this issue: [release-branch.go1.25] cmd/cgo/internal/test: use (syntactic) constant for C array bound