MsvcToolChain: Simplify logic

Remove special code dealing with msvc < 10, those did also ship a
vcvarsall.bat. Add code checking for host bitness to only show
those compilers that make sense. Also check if the batch file that
vcvarsall.bat will call actually exists, so that only installed cross
compilers are shown.

Task-number: QTCREATORBUG-8292
Change-Id: I420a0004632f02cd8a6388f2e12ba59f719909e3
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>