Fix qmake evaluation of QMAKE_TARGET.arch on msvc2010 x86_64

Fix qmake evaluation of QMAKE_TARGET.arch on msvc2010 x86_64

This change is needed because msvc2010 tools have a '\' character at
the end of environment variable VCINSTALLDIR. This variable on msvc2008
does not have this '\' character at its end. Without this change
QMAKE_TARGET.arch on msvc2010 x64 evaluates to x86 instead of x86_64.

Task-number: QTBUG-22686

Change-Id: Ifba833e9361c97568b8b3de9976023e8537b208a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>