[feature] Autodetect <target-os> from compiler triplet

[uilianries]

Proposed changes

Greetings! 👋

While working on the Conan Center boost recipe, cross-compiling required the recipe to compute and pass target-os= explicitly for every target (expected as explicitly documented at https://www.bfgroup.xyz/b2/manual/release/index.html#b2.tasks.crosscompile), even though the compiler itself already knows what it's targeting.

The gcc.jam already runs -dumpmachine, and maps the resulting triple to a <target-os> value, and registers it as a default via toolset.add-defaults.

On the other hand, clang-linux.jam and clang-darwin.jam have no equivalent, when cross-building with clang, it falls back to the host OS unless target-os= is passed.

This PR updates the clang.set-target-os-default to follow the same auto-detect rule as used in gcc.jam. I also extended gcc.jam, so both clang and gcc match in terms of OS cases.

I also added a new test to validate it, by mocking the triplet and asking b2 to deduce the target-os.

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[uilianries]

Test errors are unrelated to this PR changes:

• Could not fetch Boost module due ssl: https://dev.azure.com/bfgroup/B2/_build/results?buildId=2191&view=logs&j=8d4a1a86-36e0-5d65-af13-c0fb34aa06c3&t=e733a5ec-cba2-5068-3ac7-01a12578b416
• Execution timeout: https://ci.appveyor.com/project/bfgroup/b2/builds/54531310/job/yghbetch377h3ob1