The Xcode 'new build system' rejects empty signing identities unless
signing is explicitly marked as not allowed. Update test cases where we
turn off signing to explicitly disallow it too.
Also turn off signing in the XCTest test.
- 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
Currently the CMAKE_XCODE_EFFECTIVE_PLATFORMS property acts only as
a kind of toggle switch to enable iOS project layout features.
But instead of relying on this undocumented property, better detect
the presence of an iOS SDK directly.