ICU-22714 fix zos build, use OpenXL

[alexgubanow]

I have fixed the git clone of this repo on zos and moved build from old XLc compiler to new OpenXL compiler.
More details https://unicode-org.atlassian.net/browse/ICU-22714
As of now:

• tests are passing, some issues with files not being UTF-8 on zos, will address that in separate PR
• results of build was tested internally, we have own test suite
• following files had no BOM, need to fix tool that is used to generate those:
  icu4c/source/data/misc/currencyNumericCodes.txt
  icu4c/source/data/misc/icustd.txt
  icu4c/source/data/misc/zoneinfo64.txt

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22714
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

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[srl295]

lots of good deletion here!

For the BOMs, would it make sense just to fix them up with a build step?

[alexgubanow]

OpenXL is basically clang, that allowed me to get rid of many "OS390 only" code and tricks. Actually i have found one bug in OpenXL v2.1, it fails to generate correct name for sidedeck if output path is relative and library name has no suffix, for output library path like:
../../lib/LICU76DA
openxl v2.1 linker will generate ..x name of sidedeck
Fix for that, is already implemented, but no idea when it will be released by IBM, so i made a workaround, added "-Wl,-x$@.x" to linker command, so linker do not guess the name of sidedeck and no bug occur

I have added BOM fix in build time generated index files, also made sure those generated as UTF-8.

But regarding this files:
icu4c/source/data/misc/currencyNumericCodes.txt
icu4c/source/data/misc/icustd.txt
icu4c/source/data/misc/zoneinfo64.txt
i don't know when/who and how updates/generates it. Maybe you guys can guide me there? also i would suggest to do it in separate ticket/PR

[alexgubanow]

Hello @markusicu @srl295 what is missing for this PR to be merged?

[srl295]

@markusicu LGTM ready to merge?