x86_win64 ABI: do not use xmm0 with softfloat ABI

tests/assembly/x86_64-windows-i128-abi.rs

@@ -0,0 +1,26 @@
+//@ assembly-output: emit-asm
+//@ add-core-stubs
+//@ revisions: msvc softfloat
+//@ compile-flags: -Copt-level=3
+//@[msvc] compile-flags: --target x86_64-pc-windows-msvc
+//@[msvc] needs-llvm-components: x86
+//@[softfloat] compile-flags: --target x86_64-unknown-uefi
+//@[softfloat] needs-llvm-components: x86
+
+#![feature(no_core)]
+#![no_core]
+#![crate_type = "lib"]
+
+extern crate minicore;
+use minicore::*;
+
+// CHECK-LABEL: ret_i128
+// Hardfloat targets return via xmm0, softfloat targets via rax and rdx.
+// msvc: movaps {{.*}}, %xmm0
+// softfloat: movq (%[[INPUT:.*]]), %rax
+// softfloat-NEXT: movq 8(%[[INPUT]]), %rdx
+// CHECK-NEXT: retq
+#[no_mangle]
+pub extern "C" fn ret_i128(x: &i128) -> i128 {
+    *x
+}