The src/c++17/string-inst.cc file needs to override the default string
ABI so that it still contains the expected symbols even when the library
is configured with --with-default-libstdcxx-abi=gcc4-compatible.

	PR libstdc++/90361
	* src/c++17/string-inst.cc: Use _GLIBCXX_USE_CXX11_ABI=1 by default.

Tested x86_64-linux, committed to trunk.