configure: Put cross compile flags in EXTRA_CFLAGS

The cross compile CFLAGS are needed to properly build pixman (and any other
submodules). Only the EXTRA_CFLAGS set is passed to submodules not QEMU_CFLAGS,
so put the cross compile flags in EXTRA_CFLAGS instead.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>