`homogeneous_aggregate` does not consider empty arrays

In C, at least on aarch64-unknown-linux-gnu and powerpc64le-unknown-linux-gnu, an empty array field disqualifies a struct from being a HFA

https://godbolt.org/z/heMqY94fG

struct Yes { float a; float b;           }; 
struct No   { float a; float b; char z[0]; };  

float yes(struct Yes s) { return s.b; }
float no(struct No s) { return s.b; }

yes:
        fmov    s0, s1
        ret

no:
        lsr     x8, x0, #32
        fmov    s0, w8
        ret

Rust does not disqualify the struct when it has a zero-sized array member:

https://godbolt.org/z/W5e3asGz9

#[repr(C)]
struct Yes {
    a: f32,
    b: f32,
}

#[repr(C)]
struct No {
    a: f32,
    b: f32,
    z: [u8; 0],
}

#[unsafe(no_mangle)]
extern "C" fn yes(s: Yes) -> f32 { s.b }

#[unsafe(no_mangle)]
extern "C" fn no(s: No) -> f32 { s.b }

no:
        fmov    s0, s1
        ret

yes = no

This seems unlikely to come up in practice, given that ZSTs in C are used very infrequently.

No source file or test is named. Start by reproducing the C and Rust examples on aarch64-unknown-linux-gnu and powerpc64le-unknown-linux-gnu, then trace homogeneous_aggregate classification for zero-sized array members. Done means the ABI behavior is consistent and covered by a regression test.