Add linux-ohos (HarmonyOS) build infrastructure

[springmin]

Summary

First PR of the OpenHarmony (HarmonyOS) port series (tracking issue: #132866). This PR adds the build infrastructure for the linux-ohos target so the runtime can be cross-built with the HarmonyOS NDK. It is a no-op on all existing platforms: every change is guarded by TargetOpenHarmony (only set when the build targets linux-ohos), and CI for the standard platform matrix is expected to stay green.

Changes

File	Change
eng/build.sh	Accept -os linux-ohos; skip rootfs (NDK-based toolchain)
eng/RuntimeIdentifier.props	PortableOS=linux-ohos mapping; TargetOpenHarmony property; exclude from TargetsLinuxGlibc
eng/Subsets.props	DefaultSubsets for OHOS; UseNativeAotForComponents exclusion (crossgen2 stays IL, mirrors linux-bionic); _BuildAnyCrossArch
eng/liveBuilds.targets	CoreCLR artifacts path for linux-ohos
eng/native/build-commons.sh	OHOS NDK toolchain branch (OHOS_NDK_HOME, ohos.toolchain.cmake, OHOS_ARCH); rootfs bypass
eng/native/configureplatform.cmake	Map CMAKE_SYSTEM_NAME=OHOS to linux/musl; CLR_CMAKE_HOST_OPENHARMONY / CLR_CMAKE_TARGET_OPENHARMONY
eng/native/configurecompiler.cmake	OHOS compile flags (-Qunused-arguments, native TLS); TARGET_OPENHARMONY define (mirrors TARGET_ANDROID)
eng/native/configuretools.cmake	Find NDK tools via compiler directory (guarded to OHOS)
eng/native/gen-buildsys.sh	Rootfs bypass for OHOS cross-build
src/coreclr/clr.featuredefines.props	Disable FeatureXplatEventSource on OHOS (no LTTng in sysroot)
src/coreclr/runtime.proj, src/native/corehost/corehost.proj, src/native/libs/build-native.proj/.sh	_BuildNativeTargetOS=linux-ohos (mirrors linux-bionic)
src/native/libs/System.Native/CMakeLists.txt	Include OHOS in the "no ecm_support" platform set

Impact on existing platforms

None. TargetOpenHarmony is only defined when PortableOS == linux-ohos (i.e., when -os linux-ohos is passed). Every guarded condition either adds a new branch or excludes OHOS from an existing condition in a way that is logically equivalent for all other platforms.

Validation

• Full clr.native+libs+host+packs -os linux-ohos -arch arm64 --cross cross-build succeeds: 0 Warning(s) 0 Error(s).
• Standard platform CI matrix must pass (no-op guarantee).
• Note: HarmonyOS CI legs are not part of this PR; the follow-up PRs add sysroot compile fixes, NativeAOT support, and RID/packs (see Add OpenHarmony (HarmonyOS) support to the .NET runtime #132866 for the full series).

Note

This PR was authored with AI assistance (Copilot/agent tooling) under the repository owner's direction.

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