configure: Adjust the xgetbv instrinsic check

The 64 bit cl.exe version 16.00.30319.01 crashes with an internal
compiler error on the current check (and thus deduces it isn't
supported, even if the actual usage in libavuil/x86/cpu.c works
fine), but by assigning the value from the intrinsic to a variable,
or returning it, it works fine.

This error is fixed in cl.exe version 16.00.40219.01.

Signed-off-by: Martin Storsjö <martin@martin.st>
