* The value varies greatly whether netboot is enabled or not.
* There is no simple solution to detect has_ethmac in the linker script and set the value accordingly.
* The space check is an imperfect solution that will be superseded by stack pointer limits.
* Left commented out so we can re-enable it manually during development if stack corruption is suspected.
This also updates / is a prerequisite for updating smoltcp.
Rationale for changes made:
* compiler_builtins is now shipped in the rust prefix.
* rustc's libpanic_unwind no longer works for us because it
has a hard dependency on Box (and it's a horrible hack);
fortunately, we only ever needed a personality function
from it.
* panic and oom handlers are now set in a completely different
way.
* allocators are quite different (and finally stable).
* NLL caused internal compiler errors in runtime, so code using
NLL was rewritten to not rely on it and it was turned off.
Sebastien Bourdeauducq
whitequark