elf32-arc.c: Don't cast between function pointer and void pointer

elf32-arc.c: Don't cast between function pointer and void pointer

Casts should be avoided if at all possible, and in particular the C
standard doesn't promise that function pointers can be cast to void*
or vice-versa.  It is only mentioned under J.5 Common extensions,
saying "The following extensions are widely used in many systems, but
are not portable to all implementations."

	* elf32-arc.c (replace_func): Correct return type.
	(get_replace_function): Use a replace_func function pointer rather
	than void*.  Update associated ARC_RELOC_HOWTO define.