Every rung of the macOS SDK resolver is a read: a failing xcode-select, an
SDKROOT that names no installed SDK, or a bare directory posing as one falls
to the next rung, ending in the xcrun selection the builders used before.
An unusable SDKROOT is reported on stderr and ignored, which matches how
`xcrun --sdk macosx` treated it. SDKSettings.plist proves a directory is an
SDK, since xcrun accepts any existing path.
Native coverage builds both universal helpers with a stale SDKROOT, a plain
directory as SDKROOT, and an older installed SDK when the host has one.
Resolve SDKROOT through xcrun before passing a sysroot to clang. Exercise both real universal helper builds with default, absolute and named SDK selection, and retain resolver precedence and fallback coverage.
