[PATCH] Make cstddef / cstdarg more robust (PR 69881)

[Bernd Edlinger]

Hi,


as described in the PR 69881 it happens quite often that cstddef is
called with __need_size_t because we still support gmp-4.3.2 which
is installed by contrib/download_prerequisites.  This causes a kind
of undefined behavior.  It is just by chance that this does not cause
the gcc-6 boot-strap to fail, but it causes gcc-4.9 bootstrap to fail
when the gcc-6 version of cstddef is around.  So it looks like a
regression, because the new cstddef is more fragile than before.

As it looks like, neither c/* nor c_std/* headers will compile without
major bug-fixing first, and they are likely not in use by anybody.
Therefore I think it is best not to touch these at all, because it is
hard to test anything there.

So I propose to undefine all problematic __need_* that may cause
cstddef or cstdarg fail to compile.

Boot-strapped and reg-tested on x86_64-pc-linux-gnu
OK for trunk?


Thanks
Bernd.
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