Add OHOS (OpenHarmony / HarmonyOS) platform support

[jobor]

This adds the ohos identifier for the HamonyOS / OpenHarmony platform.

See microsoft/vcpkg#51302 and microsoft/vcpkg#51470

[Copilot]

Pull request overview

Adds first-class recognition of the ohos platform (OpenHarmony / HarmonyOS) within vcpkg’s platform expression evaluation and toolchain selection logic, enabling ports/triplets to target OHOS via VCPKG_CMAKE_SYSTEM_NAME=OHOS.

Changes:

• Add ohos as a recognized platform-expression identifier and evaluate it via VCPKG_CMAKE_SYSTEM_NAME == "OHOS".
• Route PreBuildInfo::toolchain_file() to scripts/toolchains/ohos.cmake when cmake_system_name is OHOS.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/vcpkg/platform-expression.cpp	Adds ohos identifier parsing and evaluation based on VCPKG_CMAKE_SYSTEM_NAME.
src/vcpkg/commands.build.cpp	Selects an OHOS-specific toolchain file when cmake_system_name is OHOS.

[Copilot]

The new ohos identifier behavior isn’t covered by unit tests. Since this file already has Catch2 coverage for platform-expression identifiers, please add a test case that validates ohos evaluates true when VCPKG_CMAKE_SYSTEM_NAME is OHOS (and false for a non-OHOS value).

[BillyONeal]

Do you have a screenshot or a photo of the end-to-end scenario working (e.g. vcpkg install zlib on OHOS)? We're generally happy to merge platforms someone is willing to add but because we don't know how to validate this ourselves and there are multiple parts like the toolchain that need to be in place for things to work. I think you covered all the pieces but if someone cares enough to add this hopefully they can actually try it :)

You might also want to write a PR adding this to the community supported documentation over in https://github.com/MicrosoftDocs/vcpkg-docs/blob/45b52d047aadf711d377f4e5ae6665282bb21876/vcpkg/concepts/supported-hosts.md?plain=1#L122

[jobor]

Do you have a screenshot or a photo of the end-to-end scenario working (e.g. vcpkg install zlib on OHOS)? We're generally happy to merge platforms someone is willing to add but because we don't know how to validate this ourselves and there are multiple parts like the toolchain that need to be in place for things to work. I think you covered all the pieces but if someone cares enough to add this hopefully they can actually try it :)

Sure, here's a screenshot of installing zlib with the arm64-ohos triplet:

That's with exactly this branch.

If you want, I could also provide vcpkg installation logs from Qt's CI which covers much more libraries, albeit with a few modified ports.

You might also want to write a PR adding this to the community supported documentation over in https://github.com/MicrosoftDocs/vcpkg-docs/blob/45b52d047aadf711d377f4e5ae6665282bb21876/vcpkg/concepts/supported-hosts.md?plain=1#L122

Oh right! Done here: MicrosoftDocs/vcpkg-docs#575