Add x86_64 assembly to allocate and access thread-local variables.

Add x86_64 assembly to allocate and access thread-local variables.

Thus stack_pointer, Free and Registers are now thread-local.  And
several more variables are allocated in the assembly (no longer in the
C code): heap_alloc_limit, heap_end, stack_guard, stack_start,
C_Stack_Pointer and C_Frame_Pointer.