libstdc++: Only use std::ios_base_library_init() for ELF [PR116159]

The undefined std::ios_base_library_init() symbol that is referenced by
<iostream> is only supposed to be used for targets where symbol
versioning is supported.

The mingw-w64 target defaults to --enable-symvers=gnu due to using GNU
ld but doesn't actually support symbol versioning. This means it tries
to emit references to the std::ios_base_library_init() symbol, which
isn't really defined in the library. This causes problems when using lld
to link user binaries.

Disable the undefined symbol reference for non-ELF targets.

libstdc++-v3/ChangeLog:

	PR libstdc++/116159
	* include/std/iostream (ios_base_library_init): Only define for
	ELF targets.
	* src/c++98/ios_init.cc (ios_base_library_init): Likewise.