Optimise the deletion of a QSharedPointer with no custom deleter

Optimise the deletion of a QSharedPointer with no custom deleter

When QSharedPointer is created with no user-specified custom deleter,
instead of storing a pointer in ExternalRefCount::destroyer to a
static function which, in turn, calls normalDeleter<T> indirectly (via
another function pointer), specialise the CustomDeleter class and make
it not store the pointer, but instead do the deleting directly.

The benefits are:
 - the QSharedPointer's private data is smaller
 - there is no double-indirection via indirect jumps to the actual
 deleter

Change-Id: Ice5653c144912efb1226e432267a047b9799aaca
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>