build: auto-detect ppc64 architecture

Remove the silliness of running make on powerpc like this;

$ CROSS= make

The default value of CROSS prevented this, detect the architecture in
the makefile and sent the CROSS accordingly.

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
diff --git a/make.rules b/make.rules
index dae1572..aebc4e3 100644
--- a/make.rules
+++ b/make.rules
@@ -16,7 +16,14 @@
 
 # CROSS is the prefix of your cross-compiler.
 # You can override this variable in your environment (export CROSS=...).
+ARCH := $(shell uname -p)
+
+# Auto-detect ppc64
+ifeq ($(ARCH), ppc64)
+CROSS          = ""
+else
 CROSS		?= powerpc64-linux-
+endif
 
 CELLSIZE	?= 64