ARC/GAS: Correct a `spaces' global shadowing error

ARC/GAS: Correct a `spaces' global shadowing error

Fix a commit a9752fdf8398 ("[ARC] Sync cpu names with the ones accepted
by GCC.") build regression:

cc1: warnings being treated as errors
.../gas/config/tc-arc.c: In function 'arc_show_cpu_list':
.../gas/config/tc-arc.c:3452: error: declaration of 'spaces' shadows a global declaration
.../gas/../include/libiberty.h:248: error: shadowed declaration is here
make[4]: *** [tc-arc.o] Error 1

in a way following commit 91d6fa6a035c ("Add -Wshadow to the gcc command
line options used when compiling the binutils.").

	gas/
	* config/tc-arc.c (arc_show_cpu_list): Rename `spaces' local
	variable to `space_buf'.