Add mode argument to registry access helpers allowing to access the 64bit
registry from 32bit applications and vice versa using special REGSAM
values, similar to the new QSettings formats introduced in Qt 5.7.

This allows for setting the 64bit post-mortem debugger when running
as a 32bit application on 64bit Windows. The -wow argument is extended
to indicate accessing the 64bit node from 32bit.

Task-number: QTCREATORBUG-16386
Change-Id: I7f003673777e4b8c1b259ba1905a4207b4ce0b43
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>