With more understanding of PowerPC32 ABI we've rewritten the
`VarArgPowerPC32Helper`.
New implementation fills shadow for both `reg_save_area` and
`overflow_arg_area`.
It does not copy shadow for floating-point arguments, as they are stored
in a separate space.
This implementation does not fully support passing arguments `byVal`.
This will be fixed in future PRs.
Tests were also updated via `llvm/utils/update_test_checks.py`.
7b4b43bd15