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/arm/thread-arm.cpp

@@ -594,6 +594,10 @@ Thread::condition_valid(Unsigned32 insn, Unsigned32 psr)
   return (v[insn >> 28] >> (psr >> 28)) & 1;
 }
 
+IMPLEMENT inline
+void Thread::user_single_step(bool)
+{}
+
 // ------------------------------------------------------------------------
 IMPLEMENTATION [arm && armv6plus]: