Inconsistent hashing between qint{32,64} on 32-bit platforms

Besides QTBUG-116076, and QTBUG-116077, which are probably still fixable before Qt 7, we also have that

• qHash(-42),
• qHash(-42LL)

produce different values, because we fold the upper half of the 64-bit word onto the lower half in a way that doesn't work with two's complement signed integers (it works just fine for unsigned integers).

Expectation would be, like in the epic, that equal values produce equal hash values.

This doesn't look fixable in Qt 6, as the qint32 and qint64 overloads are both inline, therefore immutable.