[patch] atomic functions on darwin

Hi,
   Some time in the last few weeks I noticed my cmake build of svn-trunk on powerpc-darwin8 start to warn about atomics being unavailable and thus building thread-unsafe. I just looked into it and found an easy solution, using the atomic functions in <libkern/OSAtomic.h> in /usr/include. The attached patch does this and also modifies the cmake and autoconf tests to 'pass' LLVM_HAS_ATOMICS. I'm able to build LLVMSupport.dylib with this patch.
   Please review, revise, criticize?

Fang

llvm-osx-atomic.patch (4.3 KB)