[committed] avoid target dependencies in a couple of tests (PR 92829)

[Martin Sebor]

PR 92829 reports failures in three -Wstringop-overflow tests on
powerpc64*.  They seem to be due to VRP determining less than
perfectly accurate range for some of the expressions used by
the tests to set them.

In g:414231ba78973dfcb11648a0a5287b989e0148bb I've committed tweaks
to the tests to avoid this problem.  Mainly for the record, I've also
raised bug 93332 for the inaccurate range info that's behind the test 
failures.

Martin
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