Rollup merge of #144541 - folkertdev:c-variadic-same-program-multiple-abis-arm, r=RalfJung,davidtwco

c-variadic: multiple ABIs in the same program for arm

similar to rust-lang/rust#144379, but for arm, requested in rust-lang/rust#144066.

Quoting rust-lang/reference#1946 (comment)

> `"aapcs"` specifically refers to the soft-float ABI where floating-point values are passed in integer registers.

However for c-variadic functions, `aapcs` behaves the same as `C`:

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#65parameter-passing

> A variadic function is always marshaled as for the base standard.

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#7the-standard-variants

> This section applies only to non-variadic functions. For a variadic function the base standard is always used both for argument passing and result return.

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I also noticed that rustc currently emit more instructions than clang for c-variadic functions on arm, see https://godbolt.org/z/hMce9rnTh. I'll fix that separately. (edit: rust-lang/rust#144549)

try-job: armhf-gnu
r? `@RalfJung`

Author: jhpratt