This patch adds two builtins (getting end-of-stack pointer and
a Boolean answer whether it was the first call to the builtin on this thread).

The idea is to replace some hard-coded values in newlib, permitting to move
later to a manually allocated stack on the compiler side without the need to
modify newlib again. The GCC patch matches what newlib did in reent; I could
imagine that we change this later on.

Lightly tested (especially by visual inspection).
Currently doing a final regtest, OK when it passes?

Any  comments to this patch - or the attached newlib patch?*

Tobias

(*) I also included a patch to newlib to see where were are heading
+ to actually use them for regtesting ...
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