Rollup merge of #141538 - folkertdev:systemv-x86_64-va_arg, r=workingjubilee

implement `va_arg` for x86_64 systemv

tracking issue: rust-lang/rust#44930

Turns out LLVM's `va_arg` is also unreliable for this target.

llvm/llvm-project#141361

So, like clang, we implement our own. I used

- the spec at https://gitlab.com/x86-psABIs/x86-64-ABI
- the clang implementation at https://github.com/llvm/llvm-project/blob/9a440f84773c56d3803f330774acb2b4f471d5b4/clang/lib/CodeGen/Targets/X86.cpp#L3041

We can take a bunch of shortcuts because the return type of `va_list` must implement `VaArgSafe`. I also extended some of the tests, because up to 11 floats can be stored in the `reg_safe_area` for this calling convention.

r? `@workingjubilee`
`@rustbot` label +F-c_variadic

try-job: x86_64-apple-1

Author: matthiaskrgr