Find a replacement for @asmcall for mark_end()/mark_start()

[Seelengrab]

Ref maleadt/LLVM.jl#320.

@asmcall may be moved around/duplicated, since it's "only" a memory clobbering sideeffect. We can't mark inline assembly as volatile in LLVM-IR so preventing the moving may be a bit harder. Perhaps a pseudo-intrinsic that we noop afterwards is suitable? That may also help with #30 .