Fix upstream ARM emulation bug that broke singlestep mode.

This fixes a really bad bug in the Thumb/Thumb2 ARM emulation related to
conditional instructions execution. The issue was that the previous
implementation did break badly if a page fault occured during the conditional
instruction's emulation. Giving an example if probably the best way to demonstrate
this. Consider the following two instructions:

    itt eq
    streq r0,[r4, #0]

These two instructions mean, respectively:
  - If the Z flag is set, execute the next instruction. Otherwise ignore it
  - Store the value of r0 at the address pointed to by r4

In single-step mode (used when debugging the emulator), each instruction is
separately JIT-ed and executed in a different pass. The 'condexec_bits' field
of the CPU state if used to store flags corresponding to the conditional
execution of up to 4 next instructions.

When the first instruction is executed, it simply sets 'condexec_bits' to a
specific value (4).

When the second instruction is executed, things get slightly bit more funky
because what happened was the following:

 - the JIT-ed code started by clearing the 'condexec_bits' right at the
   start of its sequence (a comment says "to avoid complications trying to
   do it at the end of the block", famous last words...)

 - a conditional test, based on the current value of the Z flag was added
   to skip over the rest of the instruction sequence

 - the store itself is implemented through a call to the __stl_mmu helper
   function.

The thing is that __stl_mmu may implement a *page fault* (i.e. when the
address in r4 hasn't been commited to memory yet) which requires a switch
to kernel mode (to populate the page), then going back to the instruction's
execution.

This is done in the current implementation by re-running the JIT-er for the
same instruction, however, since 'condexec_bits' was already cleared to 0,
the new JIT-ed code sequence doesn't have the conditional test to skip over
the store. The conditional instruction has been transformed into a
non-conditional one due to the page fault ! This results in either bad behaviour
or, even a crash in the emulator.

The patch fixes the clearing of condexec_bits to happen as it should, i.e.
only when execution has really cleared it.

This is preliminary work to fix the -trace option.

Also, disable the IO Thread when running the standalone emulator. This makes
debugging much easier since everything happens in a single thread.

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