x86_64: Fix to tw8573 overflow bug under -Cg option

This merge request fixes an overflow bug in the tw8573 test that occurs under Darwin or when -Cg is specified in Linux. A secondary test named tw8573a includes the -Cg and -O2 options that reproduced the error under Linux (-Cg is ignored on Windows).

Criteria

Confirm correct compilation, no test regressions, and new test passes.

Notes

Not reproducible under x86_64-win64 because -Cg is ignored on this platform, although the test should still pass.