Not all platform support C++11 thread_local. Use portable LLVM_THREAD_LOCAL macro instead.

This patch hides thread specific handling under LLVM_ENABLE_THREADS guard. It also removes usages of thread_local variables, since it has a weak support on some platforms. Instead, the patch uses single mutex for locking allocator. That may be replaced with more effective allocator later. f.e. D142318

Not all platform support C++11 thread_local. Use portableThis patch hides thread specific handling under LLVM_ENABLE_THREADS guard. LLVM_THREAD_LOCAL macro instead.It also removes usages of thread_local variables, since it has a weak support on some platforms. Instead, the patch uses single mutex for locking allocator. That may be replaced with more effective allocator later. f.e. D142318