sparc64: fix crash in ABI code for { f64, f32 } struct

This would trigger a `Size::sub: 0 - 8 would result in negative size` abort,
if `data.last_offset > offset`.

This is almost hilariously easy to trigger (https://godbolt.org/z/8rbv57xET):

```rust
#[repr(C)]
pub struct DoubleFloat {
    f: f64,
    g: f32,
}

#[no_mangle]
pub extern "C" fn foo(x: DoubleFloat) {}
```

Tests for this will be covered by the cast-target-abi.rs test added in a later commit.

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