tests/tcg: enable building for ppc64

Currently this just enables building the multiarch tests.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

tests/tcg/ppc64le/Makefile.include

+DOCKER_IMAGE=debian-ppc64el-cross
+DOCKER_CROSS_COMPILER=powerpc64le-linux-gnu-gcc