Rollup merge of #141622 - folkertdev:powerpc-va_arg, r=workingjubilee

implement `va_arg` for `powerpc`

tracking issue: https://github.com/rust-lang/rust/issues/44930

The llvm `va_arg` implementation is well-known to have serious limitations. Some planned changes to rust's `VaList` make it much more likely that LLVM miscompiles `va_arg`, so this PR adds support for the various powerpc targets. Now at least the targets that `core` has explicit support for will continue to work.

For `powerpc` (the 32-bit variant) this implementation also fixes a bug where only up to 20 variadic arguments were supported.

Locally (with qemu), these targets now pass the tests in https://github.com/rust-lang/rust/blob/master/tests/run-make/c-link-to-rust-va-list-fn/checkrust.rs. That test does not actually run for the powerpc targets in CI though.

The implementation is based on clang:

- handling of big endian architectures https://github.com/llvm/llvm-project/blob/3c8089d1ea53232d5a7cdc33f0cb43ef7d6f723b/clang/lib/CodeGen/ABIInfoImpl.cpp#L191-L193
- 64-bit https://github.com/llvm/llvm-project/blob/3c8089d1ea53232d5a7cdc33f0cb43ef7d6f723b/clang/lib/CodeGen/Targets/PPC.cpp#L969
- 32-bit https://github.com/llvm/llvm-project/blob/3c8089d1ea53232d5a7cdc33f0cb43ef7d6f723b/clang/lib/CodeGen/Targets/PPC.cpp#L430

cc `@daltenty` (target maintainer)
r? `@workingjubilee`
`@rustbot` label: +F-c_variadic

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