avx-os-support runtime-detection-fail

Affects the function »static void xgetbv(uint, uint, uint)« in file »corelib/tools/qsimd.cpp«.

The GCC and Windows compilation branches in this function are in the wrong order.

When using gcc on windows, the gcc inline asm should be used,
but instead the fallback function is used which causes Qt to think that the OS doesn't back up the ymm registers.

That causes a windows-gcc-compiled Qt with enabled avx to crash on an avx-capable-platform.

_proposed fix_: (the gcc and windows branches are switched, as in the other functions)

#ifdef Q_OS_WIN
// fallback overload in case this intrinsic does not exist: unsigned __int64 _xgetbv(unsigned int);
inline quint64 xgetbv(_int64)

#endif
static void xgetbv(uint in, uint &eax, uint &edx)
{
#if defined(Q_CC_GNU)
asm (".byte 0x0F, 0x01, 0xD0" // xgetbv instruction
: "=a" (eax), "=d" (edx)
: "c" (in));
#elif defined(Q_OS_WIN)
quint64 result = _xgetbv(in);
eax = result;
edx = result >> 32;
#endif
}

the function is the same in 5.0.2 and 5.1.0Beta1, so this applies to both