Rollup merge of #144379 - folkertdev:c-variadic-same-program-multiple-abis, r=RalfJung

test using multiple c-variadic ABIs in the same program

tracking issue: https://github.com/rust-lang/rust/issues/100189

Check that multiple c-variadic calling conventions can be used in the same program.

Clang and gcc reject defining functions with a non-default calling convention and a variable
argument list, so C programs that use multiple c-variadic calling conventions are unlikely
to come up. Here we validate that our codegen backends do in fact generate correct code.

(CI will not run this test because it runs on aarch64, I would like to at least test that this runs on windows)

try-job: `x86_64-gnu`
try-job: `x86_64-msvc-*`
try-job: `x86_64-apple-2`

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