ICU-23120 Fix malloc request larger than PTRDIFF_MAX bytes

[SoapGentoo]

only run UnicodeStringTest::TestLargeMemory() on 64-bit platforms

Checklist

• Required: Issue filed: ICU-23120
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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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[SoapGentoo]

@aheninger ping