Have QVectorData::grow, take size of header w/ padding

Have QVectorData::grow, take size of header w/ padding

This includes padding necessary to align the data array, but excludes the first
element as was done before. Size of header is the interesting piece of
information, anyway.

This simplifies calculations in a couple of places, harmonizes code with the
QRawVector fork and paves the way for further changes in QVector, namely the
memory layout.

When Q_ALIGNOF is not available, default to pointer-size alignment. This
should be honoured by malloc and won't trigger use of more expensive
aligned allocation.

Change-Id: I504022ac7595f69089cafd96e47a91b874d5771e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>