Fix non-determinism in global placement caused by different FFT output values #4518
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The global placement depends on the output of the FFT library. Without
-ffp-contract=off
the current FFT library produces different results dependent on if the compiler is targeting an x86 architecture which has the "FMA instruction".The FMA instruction omits the rounding done in the multiplication instruction, making the calculations produce different results for some input values
See https://kristerw.github.io/2021/11/09/fp-contract/
This pull request;
-ffp-contract=off
on for the whole of OpenROAD to make sure the floating point values are deterministic.