[SPARC] Zero-sized struct arguments consume a stack argument slot in clang, but not in GCC

On sparc64 GCC does not store zero-sized arguments on the stack, Clang uses a 64-bit slot for them.

https://godbolt.org/z/a6WqKjYfY

```c
struct E {};
void f(struct E a0, struct E a1, struct E a2, struct E a3,
struct E a4, struct E a5, struct E a6, unsigned char u);

void caller1(void) {
struct E e;
f(e, e, e, e, e, e, e, 0x5A);
}

void g(struct E a0, struct E a1, struct E a2, struct E a3,
struct E a4, struct E a5, struct E a6, struct E a7, unsigned char u);

void caller2(void) {
struct E e;
g(e, e, e, e, e, e, e, e, 0x5A);
}
```

On GCC the `unsigned char` is stored at the same stack offset

```assembly
caller1:
save %sp, -192, %sp
mov 90, %g1
call f, 0
stx %g1, [%sp+2223]
return %i7+8
nop
caller2:
save %sp, -192, %sp
mov 90, %g1
call g, 0
stx %g1, [%sp+2223]
return %i7+8
nop
```

But with clang the stack offset is different (by 8 bytes):

```assembly
caller1:
save %sp, -192, %sp
mov 90, %i0
call f
stx %i0, [%sp+2231]
ret
restore

caller2:
save %sp, -208, %sp
mov 90, %i0
call g
stx %i0, [%sp+2239]
ret
restore
```

Because zero-sized structs are a GNU extension, sparc64 really should follow GCC here. It's even more efficient too.