Make QStringBuilder use our qt_from_latin1 code (out-of-line)

Disassembly shows that the compiler does not perform the zero-expansion
by itself. It always opts to copy byte-by-byte, which is not very
performant.

Change-Id: I08780902461d9e3e6b7b54298f41d1eca61339c4
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>