[cxx-mem-model] compare_exchange implementation

[Andrew MacLeod <amacleod at redhat dot com>]

Here's the last of the missing intrinsics.   compare_exchange is 
slightly different than the others since it doesn't map directly to an 
rtl pattern. Its format is :

  bool __atomic_compare_exchange (T* mem, T* expected, T desired, bool 
weak, memory_order success, memory_order failure)

note that the expected parameter is a pointer as well.  In the case 
where false is returned, the value of expected is updated.

Instead of providing a complex rtl pattern to try to match this, instead 
all that is required is to provide a compare_and_swap routine, which 
this builtin will then add some wrapper code around to match the 
required functionality.

there are 2 rtl patterns it looks for,
  atomic_compare_and_swap_weak and
  atomic_compare_and_swap_strong

both are of the form

QImode atomicCAS (mem(O), current_val(O), expected(I), desired(I), 
memorymodel(const int))

When neither of these patterns are present, the implementation defaults 
to using __sync_val_compare_and_swap to provide the implementation.

New tests have been added as well.

Bootstraps and no new regressions on x86_64-unknown-linux-gnu

so a bit of different rtl hacking around in there for me... does 
everything look OK?  it seems to work :-)    Actual pattern 
implementation and debugging will happen later  when we actually write 
custom patterns for all these rather than defaulting to the current 
seq-cst patterns.

Andrew

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