x86-64 Windows fastcall ABI support.

This adds support for the "fastcall" ABI, which is the native C/C++ ABI
on Windows platforms on x86-64. It is similar to but not exactly like
System V; primarily, its argument register assignments are different,
and it requires stack shadow space.

Note that this also adjusts the handling of multi-register values in the
shared ABI implementation, and with this change, adjusts handling of
`i128`s on *both* Fastcall/x64 *and* SysV/x64 platforms. This was done
to align with actual behavior by the "rustc ABI" on both platforms, as
mapped out experimentally (Compiler Explorer link in comments). This
behavior is gated under the `enable_llvm_abi_extensions` flag.

Note also that this does *not* add x64 unwind info on Windows. That will
come in a future PR (but is planned!).

cranelift/codegen/meta/src/shared/settings.rs

@@ -216,6 +216,25 @@ pub(crate) fn define() -> SettingGroup {
         0,
     );
 
+    settings.add_bool(
+        "enable_llvm_abi_extensions",
+        r#"
+            Enable various ABI extensions defined by LLVM's behavior.
+
+            In some cases, LLVM's implementation of an ABI (calling convention)
+            goes beyond a standard and supports additional argument types or
+            behavior. This option instructs Cranelift codegen to follow LLVM's
+            behavior where applicable.
+
+            Currently, this applies only to Windows Fastcall on x86-64, and
+            allows an `i128` argument to be spread across two 64-bit integer
+            registers. The Fastcall implementation otherwise does not support
+            `i128` arguments, and will panic if they are present and this
+            option is not set.
+            "#,
+        false,
+    );
+
     // BaldrMonkey requires that not-yet-relocated function addresses be encoded
     // as all-ones bitpatterns.
     settings.add_bool(