This currently builds files in the libavcodec/x86/{vvc,h26x}
subdirectories, which is somewhat unexpected when building for
another architecture than x86.

The regular arch subdirectories are handled with

    -include $(SRC_PATH)/$(1)/$(ARCH)/Makefile

in the toplevel Makefile. Switch this to a similar optional
inclusion, using $(ARCH).
Signed-off-by: Martin Storsjö <martin@martin.st>