arm64: Enable CONFIG_64BIT for static analysis

In the Makefile there is a line that says this:

    # the checker needs the correct machine size
    CHECKFLAGS += $(if $(CONFIG_64BIT),-m64,-m32)

Set CONFIG_64BIT for ARM64 so that we pass -m64 to the static checkers
instead of -m32.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>