ICU-22787 Fix ClangCL compilation on Windows

[StefanStojanovic]

I'm working on supporting Node.js on Windows. There is an ongoing effort to enable it to be compiled with ClangCL. While working on it I noticed an issue when creating .obj file in ICU because of the *pCPU = IMAGE_FILE_MACHINE_UNKNOWN; line, and the code I made fixes that by adding a new option. Although this is a broader problem that should be fixed for multiple compilers and platforms, my change is aimed toward ClangCL and Windows, but can be used as a good foundation to work on fixes for other platforms and compilers and make corrections in future iterations.

This issue is currently our main blocker for enabling ClangCL compilation for Node.js and it'd be greatly appreciated if it could be reviewed soon.

Refs: https://github.com/nodejs/node/pull/53003

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22787
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [x] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable