c-variadic: use emit_ptr_va_arg for va_arg on sparc

[folkertdev]

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

I've built GCC for the target and validated pretty extensively that this works. Using our helper does generate worse code for i64 unfortunately, I've reported that as llvm/llvm-project#214594. But, given that this is a tier-3 target etc. I don't think it makes sense to go out of our way to do better, we can just wait for LLVM to resolve that issue.

[folkertdev]

Sparc is still gated by the c_variadic_experimental_arch feature.

The arguments to emit_ptr_va_arg here are based on the LLVM implementation. Many targets have an implementation of va_arg in Clang, but for sparc it is really LLVM that expands va_arg.

https://github.com/llvm/llvm-project/blob/5194e33faad9d84fd8d21c0305621a17e0d8a49f/llvm/lib/Target/Sparc/SparcISelLowering.cpp#L2768-L2790

• it just loads from a pointer regardless of the size of the argument, hence unconditional PassMode::Direct
• The pointer is increased by just the VT size in bytes. Due to argument promotion, the smallest type that can actually be read using va_arg is 4 bytes, and no attempt is made to align to something higher than that. So SlotSize::Bytes4.
• AllowHigherAlign::Yes would align e.g. an i128 to a 16-byte boundary. This function does not take the alignment into account at all. So, AllowHigherAlign::No
• This setting is true on BE targets with a slot size of 8, where a 4-byte value could be either in the low or high bytes. Despite being a BE target, the setting is not relevant for sparc because all values that could be passed divide cleanly into 4-byte slots.

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[folkertdev]

cc target maintainer @jonathanpallant

[folkertdev]

As context, I'd like to just stabilize c-variadic functions for this target, and for that we don't want to rely on LLVM's va_arg.