Add some very basic support for the Intel32 ABI.

In 32-bit mode, all function arguments are passed on the stack, not in
registers.

This ABI support is not complete or properly tested, but at least it
doesn't try to pass arguments in r8.

cranelift/filetests/regalloc/intel-regres.cton

@@ -38,3 +38,9 @@ ebb2(v4: i32, v5: i32, v7: i32):
 ebb3:
     return v5
 }
+
+function %select_i64(i64, i64, i32) -> i64 {
+ebb0(v0: i64, v1: i64, v2: i32):
+    v3 = select v2, v0, v1
+    return v3
+}