ARM/GAS: Correct an `index' global shadowing error

ARM/GAS: Correct an `index' global shadowing error

Fix a commit 008a97eff0ca ("[GAS][ARM]Generate unpredictable warning for
pc used in data processing instructions with register-shifted register
operand.") build regression:

cc1: warnings being treated as errors
.../gas/config/tc-arm.c: In function 'encode_arm_shift':
.../gas/config/tc-arm.c:7439: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:303: error: shadowed declaration is here
make[4]: *** [tc-arm.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-arm.c (encode_arm_shift): Rename `index' local
	variable to `op_index'.