powerpc: Fix error when cross building TAGS & cscope

Currently if I cross build TAGS or cscope from x86 I get this:
  % make ARCH=powerpc TAGS
  gcc-4.8.real: error: unrecognized command line option ‘-mbig-endian’
  GEN     TAGS
  %

I'm not setting CROSS_COMPILE= as logically I shouldn't need to and I
haven't needed to in the past when building TAGS or cscope.  Also, the
above completess correct as the error is not fatal to the build.

This was caused by:
    commit d72b08017161ab385d4ae080ea415c9eb7ceef83
    Author: Ian Munsie <imunsie@au1.ibm.com>
    powerpc: Add ability to build little endian kernels

The below fixes this by testing for the -mbig-endian option before
adding it.

I've not done the same thing in the little endian case as if
-mlittle-endian doesn't exist, we probably want to fail quickly as you
probably have an old big endian compiler.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Makefile

@@ -75,8 +75,10 @@ LDEMULATION	:= lppc
 GNUTARGET	:= powerpcle
 MULTIPLEWORD	:= -mno-multiple
 else
+ifeq ($(call cc-option-yn,-mbig-endian),y)
 override CC	+= -mbig-endian
 override AS	+= -mbig-endian
+endif
 override LD	+= -EB
 LDEMULATION	:= ppc
 GNUTARGET	:= powerpc