Add unsupported linux libc++ clang mkspec

Change-Id: Ie282196f3961777d04170fb7426f51c5fba83678
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
WinRT winmain: enable debugger waiting

WinRT applications are not invoked directly, so a debugger may attach too
late to be useful. While the application can be launched with a debugging
server, attaching directly to the running process is simpler and less
resource-intensive. For this reason, it is useful for applications to
wait for the direct debugger to attach. If the existing -qdebug parameter
is passed to the application, wait idly until the debugger attaches.

Change-Id: I7b4957beb9728ab6311b459e4d809dc5f4767780
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>