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

[gopherbot]

@dmitshur requested issue #77998 to be considered for backport to the next 1.25 minor release.

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]

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