ARC: [build] silence make defconfig warnings with host gcc 4.7

We do cross compiles for ARC Linux.
With gcc 4.7, a make defconfig spews out the following:

------------------->8--------------------------
make ARCH=arc defconfig
gcc: error: unrecognized command line option '-marc600'
gcc: error: unrecognized command line option '-mA7'
gcc: error: unrecognized command line option '-mno-sdata'
gcc: error: unrecognized command line option '-mno-mpy'
*** Default configuration is based on 'fpga_defconfig'
------------------->8--------------------------

This apparently is coming from LIBGCC line - which is strange to be
invoked for defconfig generation.
Reported-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/Makefile

@@ -8,6 +8,10 @@
 
 UTS_MACHINE := arc
 
+ifeq ($(CROSS_COMPILE),)
+CROSS_COMPILE := arc-elf32-
+endif
+
 KBUILD_DEFCONFIG := fpga_defconfig
 
 cflags-y	+= -mA7 -fno-common -pipe -fno-builtin -D__linux__