Add some sort of `-Clink-sdkroot=...` flag?

[madsmtm]

rustc uses the SDKROOT environment variable to figure out details about the SDK to pass onward to the linker (linker stubs and the SDK version). If this flag is not set, or is set for the wrong platform, we fall back to invoking xcrun --sdk $platform --show-sdk-path.

It might be beneficial to add a commandline flag to allow passing this information as well? This has been requested by Chromium in #131477 (comment), see that for motivation.

Alternatively / additionally, we could add a flag to completely opt out of passing SDK information to the linker, such as -Clinker-features=-sdkroot (partially reverting to the state before #131477). Such a flag might also be useful for e.g. cargo zigbuild (which provides the SDK internally).