[PARISC] don't unconditionally override CROSS_COMPILE for 64 bit.

The user may wish to set their own value (for real cross compiles).  Since the
top level Makefile initialises CROSS_COMPILE to empty by default, we must
check it for being empty (rather than for being defined) before we override.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>