h8300: Don't set CROSS_COMPILE unconditionally

The h8300 architecture Makefile sets the CROSS_COMPILE variable to
"h8300-unknown-linux-" unconditionally but doing that prevents the
variable to be used if a toolchain with a different binary name is
installed in the machine.

So only set CROSS_COMPILE to that default if the variable has not
been set by the user before.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>