binutils/dwarf.c: Correct an `index' global shadowing error for pre-4.8 GCC

In older gcc, shadowing a function name with a local variable name is
flagged as an error, certainly a bug but which is usually worked
around in binutils:

gcc -DHAVE_CONFIG_H -I. -I$SRC/binutils  -I. -I$SRC/binutils -I../bfd -I$SRC/binutils/../bfd -I$SRC/binutils/../include -DLOCALEDIR="\"/usr/local/share/locale\"" -Dbin_dummy_emulation=bin_vanilla_emulation  -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wshadow -Werror -I$SRC/binutils/../zlib -g -O2 -MT dwarf.o -MD -MP -MF $depbase.Tpo -c -o dwarf.o $SRC/binutils/dwarf.c &&\
	mv -f $depbase.Tpo $depbase.Po
cc1: warnings being treated as errors
$SRC/binutils/dwarf.c: In function 'display_debug_str_offsets':
$SRC/binutils/dwarf.c:6913: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:309: error: shadowed declaration is here
make[4]: *** [dwarf.o] Error 1

See also GCC PR c/53066.  This is just another one that crept in since
I and others last had to use an old version.  The name "idx" was used
in the preceding function, display_debug_addr.  Also, it was declared
c99 style (after a statement in the block).  Committed as obvious.

binutils:
	* dwarf.c (display_debug_str_offsets): Rename local variable
	index to idx.  Move to top of function.