ICU-23120 Fix malloc request larger than PTRDIFF_MAX bytes

[SoapGentoo]

• glibc does not allow allocations that exceed PTRDIFF_MAX bytes: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=9bf8e29ca136094f73f69f725f15c51facc97206
• Without this, we get compile failures on 32-bit platforms:

    work/icu/source/test/intltest/ustrtest.cpp: In member function ‘void UnicodeStringTest::TestLargeMemory()’:
    work/icu/source/test/intltest/ustrtest.cpp:2360:37: error: size ‘4294967286’ of array exceeds maximum object size ‘2147483647’
     2360 |     char16_t *buf = new char16_t[len];
          |                                     ^

Checklist

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