Use preserve_mostcc for extern "rust-cold"

As experimentation in 115242 has shown looks better than `coldcc`.

And *don't* use a different convention for cold on Windows, because that actually ends up making things worse.

cc tracking issue 97544

compiler/rustc_target/src/spec/mod.rs

@@ -2276,6 +2276,13 @@ impl Target {
             Abi::Vectorcall { .. } if ["x86", "x86_64"].contains(&&self.arch[..]) => abi,
             Abi::Fastcall { unwind } | Abi::Vectorcall { unwind } => Abi::C { unwind },
 
+            // The Windows x64 calling convention we use for `extern "Rust"`
+            // <https://learn.microsoft.com/en-us/cpp/build/x64-software-conventions#register-volatility-and-preservation>
+            // expects the callee to save `xmm6` through `xmm15`, but `PreserveMost`
+            // (that we use by default for `extern "rust-cold"`) doesn't save any of those.
+            // So to avoid bloating callers, just use the Rust convention here.
+            Abi::RustCold if self.is_like_windows && self.arch == "x86_64" => Abi::Rust,
+
             abi => abi,
         }
     }