Disable jitted Regexp for Mingw 64 bit

Work around bugs in either the jitted code, or the gcc compiler for
Windows that lead to incorrect JavaScript evaluations, most notably in
RegExp. This greatly reduces also the number of autotest failures.

Task-number: QTBUG-40814
Change-Id: I3045738c8e97b7fc4c12238f1163dd2b844ef877
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Ray Donnelly <mingw.android@gmail.com>