WinRT: Fix compiler crash (msvc2013 arm)

Visual Studio 2013 Update 3 ARM compiler has issues with the code path
trying to optimize it. Instead of disabling optimization in general,
use a pragma for the function. x86 compilation is fine though.

Change-Id: If099f7d1400cb78958d10c2fd35daef3bb75fc91
Reviewed-by: Andrew Knight <andrew.knight@digia.com>