Simplify logic for C++ exception support

exception.prf takes care of all of this.

Change-Id: I6d8ba116a425eecf94557bdd682ca58905e6263a
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Replace win32-g++ with mingw scope

Commit 773dd01 in qtbase introduced a general mingw platform scope, which
is cleaner and more flexible than matching the spec name.

(see 278152fff for a similar commit in qtbase).

Change-Id: Ia374dd565469750efed2b537e48c651fc3af8cb5
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>