fix protected mode target specification

stages/s4/protected32.json

@@ -11,6 +11,7 @@
   "linker": "rust-lld",
   "panic-strategy": "abort",
   "disable-redzone": true,
-  "features": "-mmx,-sse,+soft-float"
+  "features": "-mmx,-sse,+soft-float",
+  "rustc-abi": "x86-softfloat"
 }