X86: Single stepping

This patch enables the user land to use the CPU's single stepping mode on
x86_32 platforms. It is needed to enable the use of GDB monitor for
user-level debugging.

Was: 'foc_single_step_x86.patch'

kernel/fiasco/src/kern/thread.cpp

@@ -72,6 +72,7 @@ public:
   {
     Exr_cancel            = 0x10000,
     Exr_trigger_exception = 0x20000,
+    Exr_single_step       = 0x40000,
   };
 
   enum Vcpu_ctl_flags
@@ -139,6 +140,8 @@ public:
 
   inline Mword user_flags() const;
 
+  inline void user_single_step(bool);
+
   /** nesting level in debugger (always critical) if >1 */
   static Per_cpu<unsigned long> nested_trap_recover;
   static void handle_remote_requests_irq() asm ("handle_remote_cpu_requests");