Apple: Introduce separate system name for iOS, tvOS, and watchOS

- Remove code signing requirements for non-macOS
- Do not set deployment target for non-macOS
- Build static library for compiler feature detection for non-macOS
- Use framework to run CompilerId tests for watchOS
- Port tests to new SDK handling
- Add new Apple cross-compiling section to toolchain documentation

Closes: #17870
25 files changed