aarch64-pc-windows-msvc target needs to treat `extern "stdcall"` like `extern "C"`

[froydnj]

Compiling code like:

https://github.com/jcjones/u2f-hid-rs/blob/ea7244febb010bb8e02c5e01dd6f2b9a0082884a/src/windows/winapi.rs#L22-L49

results in:

LLVM ERROR: Unsupported calling convention

presumably because rustc attempts to translate it into an x86-specific ABI, which the AArch64 backend rightly rejects.

clang already ignores __stdcall for non-x86 targets:

https://reviews.llvm.org/D36105

I'm not completely sure what the right answer for Rust is. I think Target::adjust_abi is the rough equivalent of the above clang bit and needs to take a few more cases into account for non-x86 Windows:

rust/src/librustc_target/spec/mod.rs

Lines 764 to 776 in 31789a6

	/// Given a function ABI, turn "System" into the correct ABI for this target.
	pub fn adjust_abi(&self, abi: Abi) -> Abi {
	match abi {
	Abi::System => {
	if self.options.is_like_windows && self.arch == "x86" {
	Abi::Stdcall
	} else {
	Abi::C
	}
	},
	abi => abi
	}
	}

@alexcrichton does that sound right? If so, I'll code up a patch.

[alexcrichton]

Sounds like a good place to me!