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

[beetrees]

Fixes the 64-bit SPARC part of #115609 by replacing the current implementation with a new implementation modelled on the RISC-V calling convention code (SPARC ABI reference).

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

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[beetrees]

r? @bjorn3

[beetrees]

Note that there are currently two places that Clang and GCC disagree AFAIK:

• Clang fails to correctly align 16-byte structs when passing them as arguments, whereas GCC correctly aligns them. As the specification states that the structs must be aligned, I've filed an Clang bug at Clang: 64-bit SPARC doesn't align struct arguments as required by ABI llvm/llvm-project#144709.
• Clang passes floating point fields in packed structs in floating point register if the field is aligned within the argument as a whole, whereas GCC always passes packed structs in integer registers. The specification doesn't consider the existence of packed structs, however in my opinion Clang's interpretation seems more correct so that's what I've implemented in this PR. Issues for Rust, Clang and GCC.

[psumbera]

Thank you for doing this! My knowledge in this area is rather limited.

I was able to build latest Rust sources with your fix on top on Solaris 11.4 SPARC. Then I used this version to build Firefox on Solaris. There was no problem. Firefox also worked as expected.

I also tested some problematic cases which I was handling few years ago (https://github.com/psumbera/rust-sparc64-abi-tests). All seemed to be ok.