arch: use cross_compiling to check whether it is a cross build or not

'cross_compiling' is defined by the top Makefile and available for
arch Makefiles to check whether it is a cross build or not. A good
thing is the variable name 'cross_compiling' is self-documenting.

This is a simple replacement for m68k, mips, sh, for which $(ARCH)
and $(SRCARCH) always match.

No functional change is intended for xtensa, either.

This is rather a fix for parisc because arch/parisc/Makefile defines
UTS_MATCHINE depending on CONFIG_64BIT, therefore cc-cross-prefix
is not working in Kconfig time.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Helge Deller <deller@gmx.de>  # parisc
Acked-by: Max Filippov <jcmvbkbc@gmail.com> # xtensa
diff --git a/arch/parisc/Makefile b/arch/parisc/Makefile
index 7d9f71a..aed8ea2 100644
--- a/arch/parisc/Makefile
+++ b/arch/parisc/Makefile
@@ -41,7 +41,7 @@
 
 export LD_BFD
 
-ifneq ($(SUBARCH),$(UTS_MACHINE))
+ifdef cross_compiling
 	ifeq ($(CROSS_COMPILE),)
 		CC_SUFFIXES = linux linux-gnu unknown-linux-gnu
 		CROSS_COMPILE := $(call cc-cross-prefix, \