Use new LDC_never_intrinsic in getStackTop().

On architectures without assembler support from LLVM the getStackTop()
function must not be inlined, otherwise bad code is generated.
Add stub code for PPC64.
Fix core.atomic for compilers without D_InlineAsm_X86/_64.

The enum has64BitCAS is not defined in this case and the unittest
does not compile.