[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: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

arch/parisc/Makefile

@@ -31,7 +31,11 @@ ifdef CONFIG_64BIT
 UTS_MACHINE	:= parisc64
 CHECKFLAGS	+= -D__LP64__=1 -m64
 WIDTH		:= 64
+
+# FIXME: if no default set, should really try to locate dynamically
+ifeq ($(CROSS_COMPILE),)
 CROSS_COMPILE	:= hppa64-linux-gnu-
+endif
 else # 32-bit
 WIDTH		:=
 endif