WebAssembly: Correct an `index' global shadowing error for pre-4.8 GCC

WebAssembly: Correct an `index' global shadowing error for pre-4.8 GCC

Remove `-Wshadow' compilation errors:

cc1: warnings being treated as errors
.../bfd/wasm-module.c: In function 'wasm_scan_name_function_section':
.../bfd/wasm-module.c:312: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:303: error: shadowed declaration is here
.../bfd/wasm-module.c: In function 'wasm_register_section':
.../bfd/wasm-module.c:494: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:303: error: shadowed declaration is here
.../bfd/wasm-module.c: In function 'wasm_compute_custom_section_file_position':
.../bfd/wasm-module.c:523: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:303: error: shadowed declaration is here

and:

cc1: warnings being treated as errors
.../opcodes/wasm32-dis.c: In function 'print_insn_wasm32':
.../opcodes/wasm32-dis.c:272: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:303: error: shadowed declaration is here
make[4]: *** [wasm32-dis.lo] Error 1

which for versions of GCC before 4.8 prevent support for the WebAssembly
target from being built.  See also GCC PR c/53066.

	bfd/
	* wasm-module.c (wasm_scan_name_function_section): Rename
	`index' local variable to `idx'.

	opcodes/
	* wasm32-dis.c (print_insn_wasm32): Rename `index' local
	variable to `function_index'.