Rollup merge of #140770 - folkertdev:custom-abi, r=tgross35

add `extern "custom"` functions

tracking issue: rust-lang/rust#140829
previous discussion: rust-lang/rust#140566

In short, an `extern "custom"` function is a function with a custom ABI, that rust does not know about. Therefore, such functions can only be defined with `#[unsafe(naked)]` and `naked_asm!`, or via an `extern "C" { /* ... */ }` block. These functions cannot be called using normal rust syntax: calling them can only be done from inline assembly.

The motivation is low-level scenarios where a custom calling convention is used. Currently, we often pick `extern "C"`, but that is a lie because the function does not actually respect the C calling convention.

At the moment `"custom"` seems to be the name with the most support. That name is not final, but we need to pick something to actually implement this.

r? `@traviscross`
cc `@tgross35`

try-job: x86_64-apple-2

Author: matthiaskrgr

src/abi/mod.rs

@@ -51,6 +51,11 @@ pub(crate) fn conv_to_call_conv(
         CanonAbi::Rust | CanonAbi::C => default_call_conv,
         CanonAbi::RustCold => CallConv::Cold,
 
+        // Functions with this calling convention can only be called from assembly, but it is
+        // possible to declare an `extern "custom"` block, so the backend still needs a calling
+        // convention for declaring foreign functions.
+        CanonAbi::Custom => default_call_conv,
+
         CanonAbi::X86(x86_call) => match x86_call {
             X86Call::SysV64 => CallConv::SystemV,
             X86Call::Win64 => CallConv::WindowsFastcall,