Fixes for windows arm64 support

commit 49875dad:

openh264.recipe: Support building on Windows ARM/ARM64

commit 7318af48:

libvpx.recipe: Add support for Windows ARM64
This also adds some code for ARMv7, but that still fails elsewhere.

commit 1193919e:

cerbero: Fix dll searching on Windows
We changed the format for that var, but I forgot to change this usage.