This patch prevents the abi_symbols tool from generating symbols that
are known to occur in shared objects but must not be part of a library
ABI. This saves a bit of time during library-porting work.
However, to avoid the accidental use of ABI symbol definitions that lack
any form of manual curation, the abi_symbols tool outputs a special
message, which is explicitly checked-for by the check_abi tool.
Fixes#3112
