Build ICU with ClangCL so -march actually limits codegen

[dylan-conway]

MSVC /arch:SSE2 is a no-op on x64 and the auto-vectorizer emits AVX2/AVX512 regardless, so switch the stage 2 static rebuild to ClangCL and use /clang:-march= like we do for WebKit proper.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: c6ce5d77-68fe-426b-91a0-dfeace76e2c2

📥 Commits

Reviewing files that changed from the base of the PR and between ce9bf84 and 335daa8.

📒 Files selected for processing (1)

• build-icu.ps1

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Walkthrough

Modifies the build script to introduce ClangCL-compatible architecture flags for stage 2 builds, adds a new toolset argument configuration, and extends vcxproj patching logic to strip resource definitions and prevent rc.exe involvement during compilation.

Changes

Cohort / File(s)	Summary
Build Script Enhancement build-icu.ps1	Replaced x64 architecture flags with ClangCL-compatible flags (/clang:-march=nehalem or /clang:-march=haswell), introduced a new PlatformToolset argument for stage 2 MSBuild invocation, extended vcxproj patching to remove ResourceCompile blocks and related resource files, while preserving existing patch logic for DLL/StaticLibrary toggles and /GL disablement.

🚥 Pre-merge checks | ✅ 1 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description provides a clear technical rationale but does not follow the WebKit template structure (missing bug reference, reviewer line, detailed explanation section, and file listing).	Add a Bugzilla bug reference link, follow the template format with 'Reviewed by' line, provide detailed explanation section, and list changed files with function/method breakdowns.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main change: switching ICU builds to ClangCL to make the -march flag effective for code generation limits.

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[github-actions]

Preview Builds

Commit	Release	Date
335daa86	autobuild-preview-pr-174-335daa86	2026-03-10 05:47:12 UTC