build: fix Windows small-icu build with ClangCL

[moluopro]

When building the static libnode library with small-icu on Windows, the genccode step fails under ClangCL because the target CPU architecture is not passed to the tool.

The Windows full-ICU branch already passes -c <(target_arch) to genccode when clang==1, while preserving the existing MSVC command. The small-ICU branch did not have the same conditional handling, so it hit ICU's architecture check under ClangCL.

This change makes the Windows small-ICU branch consistent with the full-ICU branch:

• Pass -c <(target_arch) when clang==1.
• Keep the existing genccode command unchanged for the non-ClangCL MSVC path.

Reproduction

$env:config_flags = '--v8-disable-temporal-support'
.\vcbuild.bat static nonpm small-icu no-cctest openssl-no-asm x64

Windows builds for Node.js 24+ use ClangCL, and the build fails with:

genccode
genccode: CPU architecture must be set for Clang-CL
Microsoft.CppCommon.targets(254,5): error MSB8066: Custom build for '..\..\deps\icu-tmp\icudt78l.dat;..\..\out\Release\\obj\global_intermediate\icutmp\icudt78l.dat' exited with code 1. [...\tools\icu\icudata.vcxproj]

[nodejs-github-bot]

Review requested:

• @nodejs/gyp

[StefanStojanovic]

Tested it locally and it worked.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74595/