Vc: update to Vc 0.7 branch, soon 0.7.4

This is the vanilla Vc 0.7 branch. It overwrites any changes that were
committed to the Vc copy in ROOT that have not been submitted/merged
upstream.

Signed-off-by: Matthias Kretz <kretz@kde.org>
Merge from 5.34 patches:

Fix calculation of extended term for weight2 likelihood. This fixes the problem observed with getting the correct errors in weighted extended likelihood fit.
See http://root.cern.ch/phpBB3/viewtopic.php?f=15&t=16917&p=75834#p75834

Note : the version of RooNLLVar on the master was different than 5.34 patches