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

### ROOT CAUSE The issue arises due to a build constraint issue on macOS 26, where the test in question uses `// +build` constraints, which are deprecated in Go 1.26. The new syntax `//go:build` is preferred and may cause compatibility issues or build errors on newer macOS versions (like macOS 26) when used in conjunction with older Go versions (like Go 1.25). The test fails because the build system on macOS 26 expects the new syntax, but the test still uses the deprecated `// +build` syntax, leading to a build error. ### CODE FIX The fix involves updating the `// +build` constraints in the test to use the new `//go:build` syntax. This ensures compatibility with the Go 1.26 build system and avoids the build error on macOS 26. ```go // Example fix for a file with build constraints // Replace old // +build line with new //go:build line // Old code: // +build darwin // New code: // go:build darwin // Then, update any other build constraints if necessary ``` This change should be applied to all relevant files in the `cgotest` package that use build constraints.