Add warning for pointer arithmetic on pointers to void

[ehaas]

#include <stddef.h>
ptrdiff_t foo(void *a, void *b) {
    return b - a;
}

~ % gcc -Wgnu-pointer-arith test.c
test.c:3:14: warning: arithmetic on pointers to void is a GNU extension [-Wgnu-pointer-arith]
    return b - a;
           ~ ^ ~

The check can go after this line:

arocc/src/aro/Parser.zig

Line 5512 in 26324da

if (!a.ty.eql(b.ty, p.comp, false)) try p.errStr(.incompatible_pointers, tok, try p.typePairStr(a.ty, b.ty));