libstdc++: Add missing exports for ppc64le --with-long-double-format=ibm [PR105417]

The --with-long-double-abi=ibm build is missing some exports that are
present in the --with-long-double-abi=ieee build. Those symbols never
should have been exported at all, but now that they have been, they
should be exported consistently by both ibm and ieee.

This simply defines them as aliases for equivalent symbols that are
already present. The abi-tag on num_get::_M_extract_int isn't really
needed, because it only uses a std::string as a local variable, not in
the return type or function parameters, so it's safe to define the
_M_extract_int[abi:cxx11] symbols as aliases for the corresponding
function without the abi-tag.

This causes some new symbols to be added to the GLIBCXX_3.4.29 version
for the ibm long double build mode, but there is no advantage to adding
them to 3.4.30 for that build. That would just create more
inconsistencies.

libstdc++-v3/ChangeLog:

	PR libstdc++/105417
	* config/abi/post/powerpc64-linux-gnu/baseline_symbols.txt:
	Regenerate.
	* src/c++11/compatibility-ldbl-alt128.cc [_GLIBCXX_USE_DUAL_ABI]:
	Define __gnu_ieee128::num_get<C>::_M_extract_int[abi:cxx11]<I>
	symbols as aliases for corresponding symbols without abi-tag.