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.

Author: erikdesjardins

compiler/rustc_target/src/abi/call/sparc64.rs

@@ -35,12 +35,14 @@ where
         data.last_offset = data.last_offset + Reg::i32().size;
     }
 
-    for _ in 0..((offset - data.last_offset).bits() / 64)
-        .min((data.prefix.len() - data.prefix_index) as u64)
-    {
-        data.prefix[data.prefix_index] = Some(Reg::i64());
-        data.prefix_index += 1;
-        data.last_offset = data.last_offset + Reg::i64().size;
+    if data.last_offset < offset {
+        for _ in 0..((offset - data.last_offset).bits() / 64)
+            .min((data.prefix.len() - data.prefix_index) as u64)
+        {
+            data.prefix[data.prefix_index] = Some(Reg::i64());
+            data.prefix_index += 1;
+            data.last_offset = data.last_offset + Reg::i64().size;
+        }
     }
 
     if data.last_offset < offset {