This introduces a secondary flag `-sysroot` for the non-Darwin targets,
primarily Unicies. The intention here is to support a split `-sdk`,
`-sysroot` model where the `-sdk` parameter provides the Swift "SDK"
which augments the native platform's C sysroot which is indicated as
`-sysroot`. For the case of Android, this would allow us to provide a
path to the NDK sysroot and the Swift SDK allowing us to cross-compile
Android binaries from Windows.
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