Also define -os=linux for Linux and Posix docs

[ntrel]

Follow up to #22691.

Requested by @ibuclaw.

[dlang-bot]

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[ntrel]

BTW bionic/ only needs CRuntime_Bionic version ATM, maybe adding -os=linux is wrong.

[ibuclaw]

Bionic is a runtime, not a platform. Like Musl or Newlib.