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

• 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

[CLAassistant]

All committers have signed the CLA.

[aheninger]

I suspect that this change would leave this test non-functional on 64 bit platforms, which is the only place it can work in the first place. UnicodeString keeps its length as a 32 bit value. Probably the intent of the test was to make sure that UnicodeString doesn't fail in some weird way with a length near the 32 bit max limit.

PTRDIFF_MAX on 64 bit platforms won't give the right 32 bit max UnicodeString length value.

Allocating the requested sized buffer on a 32 bit platform is impossible - it's too big to fit in the available address space.

Maybe just add a condition to the existing platform #if checks to bypass the test on 32 bit platforms.