Fix WASI build (dirent, umask, errno and open flag tests)

[jrflat]

Builds successfully with swift-DEVELOPMENT-SNAPSHOT-2025-07-23-a_wasm. Tests also build but do not run (same symptom as https://forums.swift.org/t/swiftpm-fails-to-build-swift-testing-targeting-wasm/81085).

• _getConst_DT_DIR() for DT_DIR
• Modified _system_dirent struct that can be imported to Swift (WASI's dirent has variable length char d_name[] which can't be imported, so this uses NAME_MAX + 1 instead)
• #if !os(WASI) unavailable functions like umask() and pipe(), as well as unavailable Errno and open flag constants that are used directly in tests

[jrflat]

@swift-ci please test