debugger: disable the gdb fallback for MSVC compiled binary

The result of using gdb on msvc compiled binaries is not satisfactory.
It essentially shows raw disassembly only and allows switching
threads after user initiated stops. No breakpoints etc.

It is expected that people would never use the msvc/gdb combination
consciously so it seems better to not provide the feature at all.

Change-Id: I8918103e44643bfa1ad0b419a911a232f8a5299e
Task-number: QTCREATORBUG-7309
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: David Schulz <david.schulz@nokia.com>