[core.sys docs] Build HTML with -os=platform flag

[ntrel]

Ignore Posix and linux as Linux is used to build docs already.

The generated files aren't linked to yet - that needs dlang/dlang.org#4412.

[dlang-bot]

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Testing this PR locally

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[ntrel]

Maybe cross-compilation for doc generation would work?

[ntrel]

• There isn't an -os= flag for core.sys.bionic, I'm not sure how to set CRuntime_Bionic.
• There isn't an -os=netbsd flag (for some reason) for NetBSD version.

[ntrel]

This lists some of the changed HTML files:
https://dtest.dlang.io/results/6e5c91db42517e7e53b828ecd3f0d9046ec34447/6ace42403201c63f8a22758312dcbd22c4827114/