Add support for emitting functions with coldcc in LLVM

The eventual goal is to try using this for things like the internal panicking stuff, to see whether it helps.

compiler/rustc_codegen_cranelift/src/abi/mod.rs

@@ -22,7 +22,7 @@ fn clif_sig_from_fn_abi<'tcx>(
     fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
 ) -> Signature {
     let call_conv = match fn_abi.conv {
-        Conv::Rust | Conv::C => default_call_conv,
+        Conv::Rust | Conv::C | Conv::RustCold => default_call_conv,
         Conv::X86_64SysV => CallConv::SystemV,
         Conv::X86_64Win64 => CallConv::WindowsFastcall,
         Conv::ArmAapcs