Rollup merge of rust-lang#142680 - beetrees:sparc64-float-struct-abi, r=tgross35

Fix passing/returning structs with the 64-bit SPARC ABI

Fixes the 64-bit SPARC part of rust-lang#115609 by replacing the current implementation with a new implementation modelled on the RISC-V calling convention code ([SPARC ABI reference](https://sparc.org/wp-content/uploads/2014/01/SCD.2.4.1.pdf.gz)).

Pinging `sparcv9-sun-solaris` target maintainers: @psumbera @kulikjak
Fixes rust-lang#115336
Fixes rust-lang#115399
Fixes rust-lang#122620
Fixes rust-lang#147883
r? @workingjubilee

Author: Zalathar

compiler/rustc_abi/src/callconv/reg.rs

@@ -35,6 +35,7 @@ impl Reg {
 
     reg_ctor!(f32, Float, 32);
     reg_ctor!(f64, Float, 64);
+    reg_ctor!(f128, Float, 128);
 }
 
 impl Reg {