Split the ENABLE_THREADS #define in two. Now ENABLE_THREADS indicates the ability...

Split the ENABLE_THREADS #define in two.  Now ENABLE_THREADS indicates the ability to run multiple threads at once in the JIT, and requires only
mutex support.  LLVM_MULTITHREADED indicates (or will indicate) the ability to run LLVM itself across multiple threads, and requires atomics support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72140 91177308-0d34-0410-b5e6-96231b3b80d8