c-variadic: fix for sparc64
tracking issue: https://github.com/rust-lang/rust/issues/44930
Turns out it's a big-endian target that right-adjusts values in the stack slots.
Apparently these tests do get run occasionally, though i don't think test failures are usually turned into issues on this repo. I guess we could add an assembly test here too, though really you just have to run these tests. I've tried this locally with qemu, and it passes all c-variadic tests.
cc @thejpster @glaubitz
r? tgross35
Trevor Gross
85468d1fdb