S12Z/opcodes: Correct a `reg' global shadowing error for pre-4.8 GCC

S12Z/opcodes: Correct a `reg' global shadowing error for pre-4.8 GCC

Remove `-Wshadow' compilation errors:

cc1: warnings being treated as errors
.../opcodes/s12z-dis.c: In function 'lea_reg_xys_opr':
.../opcodes/s12z-dis.c:814: error: declaration of 'reg' shadows a global declaration
.../opcodes/s12z-dis.c:783: error: shadowed declaration is here
.../opcodes/s12z-dis.c: In function 'lea_reg_xys':
.../opcodes/s12z-dis.c:843: error: declaration of 'reg' shadows a global declaration
.../opcodes/s12z-dis.c:783: error: shadowed declaration is here
.../opcodes/s12z-dis.c: In function 'print_insn_loop_primitive':
.../opcodes/s12z-dis.c:2206: error: declaration of 'reg' shadows a global declaration
.../opcodes/s12z-dis.c:783: error: shadowed declaration is here

which for versions of GCC before 4.8 prevent support for S12Z targets
from being built.  See also GCC PR c/53066.

	opcodes/
	* s12z-dis.c (lea_reg_xys_opr): Rename `reg' local variable to
	`reg_xys'.
	(lea_reg_xys): Likewise.
	(print_insn_loop_primitive): Rename `reg' local variable to
	`reg_dxy'.