CI ARM: do not build mmal on arm64

MMAL headers not present in 64-bit RaspiOS 11 libraspberrypi-dev (but
mmal.pc is still there!)

The MMAL API doesn't seem to be longer supported, see necro-posting
in GH-129.

.github/scripts/Linux/arm/build.sh

@@ -18,7 +18,7 @@ set -- "$@" --enable-decklink --enable-file --enable-ndi --enable-rtsp \
 set -- "$@" --enable-caca --enable-gl-display --enable-panogl_disp --enable-sdl                         # display
 set -- "$@" --enable-libavcodec --enable-rtdxt --enable-libswscale --enable-uyvy                        # compression
 set -- "$@" --enable-blank --enable-holepunch --enable-natpmp --enable-pcp --enable-resize --enable-scale --enable-sdp-http --enable-testcard-extras --enable-text --enable-video-mixer --enable-zfec # extras (pp. etc)
-if [ "$ARCH" = armhf ]; then # Raspbian
+if [ "$ARCH" = armhf ]; then # 64b doesn't have needed headers
         set -- "$@" --enable-lavc-hw-accel-rpi4
 fi
 set -- "$@" --disable-vulkan

configure.ac

@@ -1588,6 +1588,7 @@ if test $lavc_hwacc_rpi4_req != no; then
         PKG_CHECK_MODULES([BCM_HOST], [bcm_host], [ ], [ FOUND_HWACC_RPI4_DEP=no ])
         PKG_CONFIG_PATH=$SAVED_PKG_CONFIG_PATH
         AC_CHECK_HEADER([libavcodec/rpi_zc.h], [  ], [FOUND_HWACC_RPI4_DEP=no], [#include <libavcodec/avcodec.h>])
+        AC_CHECK_HEADER([interface/mmal/mmal.h], [], [FOUND_HWACC_RPI4_DEP=no])
         if test "$FOUND_HWACC_RPI4_DEP" = yes; then
                 LAVC_HWACC_FLAGS="${LAVC_HWACC_FLAGS} -DHWACC_RPI4 ${MMAL_CFLAGS} ${BCM_HOST_CFLAGS}"
                 LAVC_HWACC_LIBS="${LAVC_HWACC_LIBS} ${MMAL_LIBS} ${BCM_HOST_LIBS}"